< draft-ietf-tls-protocol   rfc2246.txt 
Transport Layer Security Working Group Tim Dierks
INTERNET-DRAFT Certicom
Expires May 12, 1999 Christopher Allen
Certicom
November 12, 1998
The TLS Protocol Network Working Group T. Dierks
Version 1.0 Request for Comments: 2246 Certicom
Category: Standards Track C. Allen
Certicom
January 1999
<draft-ietf-tls-protocol-06.txt> The TLS Protocol
Version 1.0
Status of this memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document specifies an Internet standards track protocol for the
documents of the Internet Engineering Task Force (IETF), its areas, Internet community, and requests discussion and suggestions for
and its working groups. Note that other groups may also distribute improvements. Please refer to the current edition of the "Internet
working documents as Internet-Drafts. Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Internet-Drafts are draft documents valid for a maximum of six Copyright Notice
months and may be updated, replaced, or made obsolete by other
documents at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as work in progress.
To learn the current status of any Internet-Draft, please check the Copyright (C) The Internet Society (1999). All Rights Reserved.
1id-abstracts.txt listing contained in the Internet Drafts Shadow
Directories on ftp.ietf.org (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Abstract Abstract
This document specifies Version 1.0 of the Transport Layer Security This document specifies Version 1.0 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications privacy (TLS) protocol. The TLS protocol provides communications privacy over
over the Internet. The protocol allows client/server applications to the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping, communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery. tampering, or message forgery.
Table of Contents Table of Contents
Status of this memo 1
Abstract 1
Table of Contents 2
1. Introduction 3 1. Introduction 3
2. Goals 4 2. Goals 4
3. Goals of this document 5 3. Goals of this document 5
4. Presentation language 5 4. Presentation language 5
4.1. Basic block size 6 4.1. Basic block size 6
4.2. Miscellaneous 6 4.2. Miscellaneous 6
4.3. Vectors 6 4.3. Vectors 6
4.4. Numbers 7 4.4. Numbers 7
4.5. Enumerateds 7 4.5. Enumerateds 7
4.6. Constructed types 8 4.6. Constructed types 8
4.6.1. Variants 8 4.6.1. Variants 9
4.7. Cryptographic attributes 9 4.7. Cryptographic attributes 10
4.8. Constants 11 4.8. Constants 11
5. HMAC and the pseudorandom function 11 5. HMAC and the pseudorandom function 11
6. The TLS Record Protocol 13 6. The TLS Record Protocol 13
6.1. Connection states 13 6.1. Connection states 14
6.2. Record layer 16 6.2. Record layer 16
6.2.1. Fragmentation 16 6.2.1. Fragmentation 16
6.2.2. Record compression and decompression 17 6.2.2. Record compression and decompression 17
6.2.3. Record payload protection 17 6.2.3. Record payload protection 18
6.2.3.1. Null or standard stream cipher 18 6.2.3.1. Null or standard stream cipher 19
6.2.3.2. CBC block cipher 18 6.2.3.2. CBC block cipher 19
6.3. Key calculation 20 6.3. Key calculation 21
6.3.1. Export key generation example 21 6.3.1. Export key generation example 22
7. The TLS Handshake Protocol 22 7. The TLS Handshake Protocol 23
7.1. Change cipher spec protocol 22 7.1. Change cipher spec protocol 24
7.2. Alert protocol 23 7.2. Alert protocol 24
7.2.1. Closure alerts 24 7.2.1. Closure alerts 25
7.2.2. Error alerts 24 7.2.2. Error alerts 26
7.3. Handshake Protocol overview 27 7.3. Handshake Protocol overview 29
7.4. Handshake protocol 30 7.4. Handshake protocol 32
7.4.1. Hello messages 31 7.4.1. Hello messages 33
7.4.1.1. Hello request 31 7.4.1.1. Hello request 33
7.4.1.2. Client hello 31 7.4.1.2. Client hello 34
7.4.1.3. Server hello 34 7.4.1.3. Server hello 36
7.4.2. Server certificate 35 7.4.2. Server certificate 37
7.4.3. Server key exchange message 36 7.4.3. Server key exchange message 39
7.4.4. Certificate request 39 7.4.4. Certificate request 41
7.4.5. Server hello done 39 7.4.5. Server hello done 42
7.4.6. Client certificate 40 7.4.6. Client certificate 43
7.4.7. Client key exchange message 40 7.4.7. Client key exchange message 43
7.4.7.1. RSA encrypted premaster secret message 41 7.4.7.1. RSA encrypted premaster secret message 44
7.4.7.2. Client Diffie-Hellman public value 42 7.4.7.2. Client Diffie-Hellman public value 45
7.4.8. Certificate verify 42 7.4.8. Certificate verify 45
7.4.9. Finished 43 7.4.9. Finished 46
8. Cryptographic computations 44 8. Cryptographic computations 47
8.1. Computing the master secret 44 8.1. Computing the master secret 47
8.1.1. RSA 44 8.1.1. RSA 48
8.1.2. Diffie-Hellman 45 8.1.2. Diffie-Hellman 48
9. Mandatory Cipher Suites 45 9. Mandatory Cipher Suites 48
10. Application data protocol 45 10. Application data protocol 48
A. Protocol constant values 45 A. Protocol constant values 49
A.1. Record layer 45 A.1. Record layer 49
A.2. Change cipher specs message 46 A.2. Change cipher specs message 50
A.3. Alert messages 46 A.3. Alert messages 50
A.4. Handshake protocol 47 A.4. Handshake protocol 51
A.4.1. Hello messages 48 A.4.1. Hello messages 51
A.4.2. Server authentication and key exchange messages 48 A.4.2. Server authentication and key exchange messages 52
A.4.3. Client authentication and key exchange messages 49 A.4.3. Client authentication and key exchange messages 53
A.4.4. Handshake finalization message 50 A.4.4. Handshake finalization message 54
A.5. The CipherSuite 50 A.5. The CipherSuite 54
A.6. The Security Parameters 52 A.6. The Security Parameters 56
B. Glossary 52 B. Glossary 57
C. CipherSuite definitions 56 C. CipherSuite definitions 61
D. Implementation Notes 58 D. Implementation Notes 64
D.1. Temporary RSA keys 58 D.1. Temporary RSA keys 64
D.2. Random Number Generation and Seeding 59 D.2. Random Number Generation and Seeding 64
D.3. Certificates and authentication 59 D.3. Certificates and authentication 65
D.4. CipherSuites 59 D.4. CipherSuites 65
E. Backward Compatibility With SSL 60 E. Backward Compatibility With SSL 66
E.1. Version 2 client hello 61 E.1. Version 2 client hello 67
E.2. Avoiding man-in-the-middle version rollback 62 E.2. Avoiding man-in-the-middle version rollback 68
F. Security analysis 62 F. Security analysis 69
F.1. Handshake protocol 63 F.1. Handshake protocol 69
F.1.1. Authentication and key exchange 63 F.1.1. Authentication and key exchange 69
F.1.1.1. Anonymous key exchange 63 F.1.1.1. Anonymous key exchange 69
F.1.1.2. RSA key exchange and authentication 64 F.1.1.2. RSA key exchange and authentication 70
F.1.1.3. Diffie-Hellman key exchange with authentication 64 F.1.1.3. Diffie-Hellman key exchange with authentication 71
F.1.2. Version rollback attacks 65 F.1.2. Version rollback attacks 71
F.1.3. Detecting attacks against the handshake protocol 65 F.1.3. Detecting attacks against the handshake protocol 72
F.1.4. Resuming sessions 65 F.1.4. Resuming sessions 72
F.1.5. MD5 and SHA 66 F.1.5. MD5 and SHA 72
F.2. Protecting application data 66 F.2. Protecting application data 72
F.3. Final notes 66 F.3. Final notes 73
G. Patent Statement 67 G. Patent Statement 74
References 68 Security Considerations 75
Credits 71 References 75
Comments 72 Credits 77
Comments 78
Full Copyright Statement 80
1. Introduction 1. Introduction
The primary goal of the TLS Protocol is to provide privacy and data The primary goal of the TLS Protocol is to provide privacy and data
integrity between two communicating applications. The protocol is integrity between two communicating applications. The protocol is
composed of two layers: the TLS Record Protocol and the TLS composed of two layers: the TLS Record Protocol and the TLS Handshake
Handshake Protocol. At the lowest level, layered on top of some Protocol. At the lowest level, layered on top of some reliable
reliable transport protocol (e.g., TCP[TCP]), is the TLS Record transport protocol (e.g., TCP[TCP]), is the TLS Record Protocol. The
Protocol. The TLS Record Protocol provides connection security that TLS Record Protocol provides connection security that has two basic
has two basic properties: properties:
- The connection is private. Symmetric cryptography is used for - The connection is private. Symmetric cryptography is used for
data encryption (e.g., DES[DES], RC4[RC4], etc.) The keys for data encryption (e.g., DES [DES], RC4 [RC4], etc.) The keys for
this symmetric encryption are generated uniquely for each this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record protocol (such as the TLS Handshake Protocol). The Record
Protocol can also be used without encryption. Protocol can also be used without encryption.
- The connection is reliable. Message transport includes a message - The connection is reliable. Message transport includes a message
integrity check using a keyed MAC. Secure hash functions (e.g., integrity check using a keyed MAC. Secure hash functions (e.g.,
SHA, MD5, etc.) are used for MAC computations. The Record SHA, MD5, etc.) are used for MAC computations. The Record
Protocol can operate without a MAC, but is generally only used Protocol can operate without a MAC, but is generally only used in
in this mode while another protocol is using the Record Protocol this mode while another protocol is using the Record Protocol as
as a transport for negotiating security parameters. a transport for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of various higher The TLS Record Protocol is used for encapsulation of various higher
level protocols. One such encapsulated protocol, the TLS Handshake level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other Protocol, allows the server and client to authenticate each other and
and to negotiate an encryption algorithm and cryptographic keys to negotiate an encryption algorithm and cryptographic keys before
before the application protocol transmits or receives its first byte the application protocol transmits or receives its first byte of
of data. The TLS Handshake Protocol provides connection security data. The TLS Handshake Protocol provides connection security that
that has three basic properties: has three basic properties:
- The peer's identity can be authenticated using asymmetric, or - The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA[RSA], DSS[DSS], etc.). This public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
authentication can be made optional, but is generally required authentication can be made optional, but is generally required
for at least one of the peers. for at least one of the peers.
- The negotiation of a shared secret is secure: the negotiated - The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any secret is unavailable to eavesdroppers, and for any authenticated
authenticated connection the secret cannot be obtained, even by connection the secret cannot be obtained, even by an attacker who
an attacker who can place himself in the middle of the can place himself in the middle of the connection.
connection.
- The negotiation is reliable: no attacker can modify the - The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties negotiation communication without being detected by the parties
to the communication. to the communication.
One advantage of TLS is that it is application protocol independent. One advantage of TLS is that it is application protocol independent.
Higher level protocols can layer on top of the TLS Protocol Higher level protocols can layer on top of the TLS Protocol
transparently. The TLS standard, however, does not specify how transparently. The TLS standard, however, does not specify how
protocols add security with TLS; the decisions on how to initiate protocols add security with TLS; the decisions on how to initiate TLS
TLS handshaking and how to interpret the authentication certificates handshaking and how to interpret the authentication certificates
exchanged are left up to the judgment of the designers and exchanged are left up to the judgment of the designers and
implementors of protocols which run on top of TLS. implementors of protocols which run on top of TLS.
2. Goals 2. Goals
The goals of TLS Protocol, in order of their priority, are: The goals of TLS Protocol, in order of their priority, are:
1. Cryptographic security: TLS should be used to establish a secure 1. Cryptographic security: TLS should be used to establish a secure
connection between two parties. connection between two parties.
2. Interoperability: Independent programmers should be able to 2. Interoperability: Independent programmers should be able to
develop applications utilizing TLS that will then be able to develop applications utilizing TLS that will then be able to
successfully exchange cryptographic parameters without knowledge successfully exchange cryptographic parameters without knowledge
of one another's code. of one another's code.
3. Extensibility: TLS seeks to provide a framework into which new 3. Extensibility: TLS seeks to provide a framework into which new
public key and bulk encryption methods can be incorporated as public key and bulk encryption methods can be incorporated as
necessary. This will also accomplish two sub-goals: to prevent necessary. This will also accomplish two sub-goals: to prevent
the need to create a new protocol (and risking the introduction the need to create a new protocol (and risking the introduction
of possible new weaknesses) and to avoid the need to implement of possible new weaknesses) and to avoid the need to implement an
an entire new security library. entire new security library.
4. Relative efficiency: Cryptographic operations tend to be highly 4. Relative efficiency: Cryptographic operations tend to be highly
CPU intensive, particularly public key operations. For this CPU intensive, particularly public key operations. For this
reason, the TLS protocol has incorporated an optional session reason, the TLS protocol has incorporated an optional session
caching scheme to reduce the number of connections that need to caching scheme to reduce the number of connections that need to
be established from scratch. Additionally, care has been taken be established from scratch. Additionally, care has been taken to
to reduce network activity. reduce network activity.
3. Goals of this document 3. Goals of this document
This document and the TLS protocol itself are based on the SSL 3.0 This document and the TLS protocol itself are based on the SSL 3.0
Protocol Specification as published by Netscape. The differences Protocol Specification as published by Netscape. The differences
between this protocol and SSL 3.0 are not dramatic, but they are between this protocol and SSL 3.0 are not dramatic, but they are
significant enough that TLS 1.0 and SSL 3.0 do not interoperate significant enough that TLS 1.0 and SSL 3.0 do not interoperate
(although TLS 1.0 does incorporate a mechanism by which a TLS (although TLS 1.0 does incorporate a mechanism by which a TLS
implementation can back down to SSL 3.0). This document is intended implementation can back down to SSL 3.0). This document is intended
primarily for readers who will be implementing the protocol and primarily for readers who will be implementing the protocol and those
those doing cryptographic analysis of it. The specification has been doing cryptographic analysis of it. The specification has been
written with this in mind, and it is intended to reflect the needs written with this in mind, and it is intended to reflect the needs of
of those two groups. For that reason, many of the those two groups. For that reason, many of the algorithm-dependent
algorithm-dependent data structures and rules are included in the data structures and rules are included in the body of the text (as
body of the text (as opposed to in an appendix), providing easier opposed to in an appendix), providing easier access to them.
access to them.
This document is not intended to supply any details of service This document is not intended to supply any details of service
definition nor interface definition, although it does cover select definition nor interface definition, although it does cover select
areas of policy as they are required for the maintenance of solid areas of policy as they are required for the maintenance of solid
security. security.
4. Presentation language 4. Presentation language
This document deals with the formatting of data in an external This document deals with the formatting of data in an external
representation. The following very basic and somewhat casually representation. The following very basic and somewhat casually
defined presentation syntax will be used. The syntax draws from defined presentation syntax will be used. The syntax draws from
several sources in its structure. Although it resembles the several sources in its structure. Although it resembles the
programming language "C" in its syntax and XDR [XDR] in both its programming language "C" in its syntax and XDR [XDR] in both its
syntax and intent, it would be risky to draw too many parallels. The syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document TLS only, not purpose of this presentation language is to document TLS only, not to
to have general application beyond that particular goal. have general application beyond that particular goal.
4.1. Basic block size 4.1. Basic block size
The representation of all data items is explicitly specified. The The representation of all data items is explicitly specified. The
basic data block size is one byte (i.e. 8 bits). Multiple byte data basic data block size is one byte (i.e. 8 bits). Multiple byte data
items are concatenations of bytes, from left to right, from top to items are concatenations of bytes, from left to right, from top to
bottom. From the bytestream a multi-byte item (a numeric in the bottom. From the bytestream a multi-byte item (a numeric in the
example) is formed (using C notation) by: example) is formed (using C notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
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A vector (single dimensioned array) is a stream of homogeneous data A vector (single dimensioned array) is a stream of homogeneous data
elements. The size of the vector may be specified at documentation elements. The size of the vector may be specified at documentation
time or left unspecified until runtime. In either case the length time or left unspecified until runtime. In either case the length
declares the number of bytes, not the number of elements, in the declares the number of bytes, not the number of elements, in the
vector. The syntax for specifying a new type T' that is a fixed vector. The syntax for specifying a new type T' that is a fixed
length vector of type T is length vector of type T is
T T'[n]; T T'[n];
Here T' occupies n bytes in the data stream, where n is a multiple Here T' occupies n bytes in the data stream, where n is a multiple of
of the size of T. The length of the vector is not included in the the size of T. The length of the vector is not included in the
encoded stream. encoded stream.
In the following example, Datum is defined to be three consecutive In the following example, Datum is defined to be three consecutive
bytes that the protocol does not interpret, while Data is three bytes that the protocol does not interpret, while Data is three
consecutive Datum, consuming a total of nine bytes. consecutive Datum, consuming a total of nine bytes.
opaque Datum[3]; /* three uninterpreted bytes */ opaque Datum[3]; /* three uninterpreted bytes */
Datum Data[9]; /* 3 consecutive 3 byte vectors */ Datum Data[9]; /* 3 consecutive 3 byte vectors */
Variable length vectors are defined by specifying a subrange of Variable length vectors are defined by specifying a subrange of legal
legal lengths, inclusively, using the notation <floor..ceiling>. lengths, inclusively, using the notation <floor..ceiling>. When
When encoded, the actual length precedes the vector's contents in encoded, the actual length precedes the vector's contents in the byte
the byte stream. The length will be in the form of a number stream. The length will be in the form of a number consuming as many
consuming as many bytes as required to hold the vector's specified bytes as required to hold the vector's specified maximum (ceiling)
maximum (ceiling) length. A variable length vector with an actual length. A variable length vector with an actual length field of zero
length field of zero is referred to as an empty vector. is referred to as an empty vector.
T T'<floor..ceiling>; T T'<floor..ceiling>;
In the following example, mandatory is a vector that must contain In the following example, mandatory is a vector that must contain
between 300 and 400 bytes of type opaque. It can never be empty. The between 300 and 400 bytes of type opaque. It can never be empty. The
actual length field consumes two bytes, a uint16, sufficient to actual length field consumes two bytes, a uint16, sufficient to
represent the value 400 (see Section 4.4). On the other hand, longer represent the value 400 (see Section 4.4). On the other hand, longer
can represent up to 800 bytes of data, or 400 uint16 elements, and can represent up to 800 bytes of data, or 400 uint16 elements, and it
it may be empty. Its encoding will include a two byte actual length may be empty. Its encoding will include a two byte actual length
field prepended to the vector. The length of an encoded vector must field prepended to the vector. The length of an encoded vector must
be an even multiple of the length of a single element (for example, be an even multiple of the length of a single element (for example, a
a 17 byte vector of uint16 would be illegal). 17 byte vector of uint16 would be illegal).
opaque mandatory<300..400>; opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */ /* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>; uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */ /* zero to 400 16-bit unsigned integers */
4.4. Numbers 4.4. Numbers
The basic numeric data type is an unsigned byte (uint8). All larger The basic numeric data type is an unsigned byte (uint8). All larger
numeric data types are formed from fixed length series of bytes numeric data types are formed from fixed length series of bytes
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All values, here and elsewhere in the specification, are stored in All values, here and elsewhere in the specification, are stored in
"network" or "big-endian" order; the uint32 represented by the hex "network" or "big-endian" order; the uint32 represented by the hex
bytes 01 02 03 04 is equivalent to the decimal value 16909060. bytes 01 02 03 04 is equivalent to the decimal value 16909060.
4.5. Enumerateds 4.5. Enumerateds
An additional sparse data type is available called enum. A field of An additional sparse data type is available called enum. A field of
type enum can only assume the values declared in the definition. type enum can only assume the values declared in the definition.
Each definition is a different type. Only enumerateds of the same Each definition is a different type. Only enumerateds of the same
type may be assigned or compared. Every element of an enumerated type may be assigned or compared. Every element of an enumerated must
must be assigned a value, as demonstrated in the following example. be assigned a value, as demonstrated in the following example. Since
Since the elements of the enumerated are not ordered, they can be the elements of the enumerated are not ordered, they can be assigned
assigned any unique value, in any order. any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te; enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
Enumerateds occupy as much space in the byte stream as would its Enumerateds occupy as much space in the byte stream as would its
maximal defined ordinal value. The following definition would cause maximal defined ordinal value. The following definition would cause
one byte to be used to carry fields of type Color. one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color; enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to One may optionally specify a value without its associated tag to
force the width definition without defining a superfluous element. force the width definition without defining a superfluous element.
In the following example, Taste will consume two bytes in the data In the following example, Taste will consume two bytes in the data
stream but can only assume the values 1, 2 or 4. stream but can only assume the values 1, 2 or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste; enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the The names of the elements of an enumeration are scoped within the
defined type. In the first example, a fully qualified reference to defined type. In the first example, a fully qualified reference to
the second element of the enumeration would be Color.blue. Such the second element of the enumeration would be Color.blue. Such
qualification is not required if the target of the assignment is qualification is not required if the target of the assignment is well
well specified. specified.
Color color = Color.blue; /* overspecified, legal */ Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */ Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation, For enumerateds that are never converted to external representation,
the numerical information may be omitted. the numerical information may be omitted.
enum { low, medium, high } Amount; enum { low, medium, high } Amount;
4.6. Constructed types 4.6. Constructed types
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syntax for definition is much like that of C. syntax for definition is much like that of C.
struct { struct {
T1 f1; T1 f1;
T2 f2; T2 f2;
... ...
Tn fn; Tn fn;
} [[T]]; } [[T]];
The fields within a structure may be qualified using the type's name The fields within a structure may be qualified using the type's name
using a syntax much like that available for enumerateds. For using a syntax much like that available for enumerateds. For example,
example, T.f2 refers to the second field of the previous T.f2 refers to the second field of the previous declaration.
declaration. Structure definitions may be embedded. Structure definitions may be embedded.
4.6.1. Variants 4.6.1. Variants
Defined structures may have variants based on some knowledge that is Defined structures may have variants based on some knowledge that is
available within the environment. The selector must be an enumerated available within the environment. The selector must be an enumerated
type that defines the possible variants the structure defines. There type that defines the possible variants the structure defines. There
must be a case arm for every element of the enumeration declared in must be a case arm for every element of the enumeration declared in
the select. The body of the variant structure may be given a label the select. The body of the variant structure may be given a label
for reference. The mechanism by which the variant is selected at for reference. The mechanism by which the variant is selected at
runtime is not prescribed by the presentation language. runtime is not prescribed by the presentation language.
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is a narrowed type of a VariantRecord containing a variant_body of is a narrowed type of a VariantRecord containing a variant_body of
type V2. type V2.
4.7. Cryptographic attributes 4.7. Cryptographic attributes
The four cryptographic operations digital signing, stream cipher The four cryptographic operations digital signing, stream cipher
encryption, block cipher encryption, and public key encryption are encryption, block cipher encryption, and public key encryption are
designated digitally-signed, stream-ciphered, block-ciphered, and designated digitally-signed, stream-ciphered, block-ciphered, and
public-key-encrypted, respectively. A field's cryptographic public-key-encrypted, respectively. A field's cryptographic
processing is specified by prepending an appropriate key word processing is specified by prepending an appropriate key word
designation before the field's type specification. Cryptographic designation before the field's type specification. Cryptographic keys
keys are implied by the current session state (see Section 6.1). are implied by the current session state (see Section 6.1).
In digital signing, one-way hash functions are used as input for a In digital signing, one-way hash functions are used as input for a
signing algorithm. A digitally-signed element is encoded as an signing algorithm. A digitally-signed element is encoded as an opaque
opaque vector <0..2^16-1>, where the length is specified by the vector <0..2^16-1>, where the length is specified by the signing
signing algorithm and key. algorithm and key.
In RSA signing, a 36-byte structure of two hashes (one SHA and one In RSA signing, a 36-byte structure of two hashes (one SHA and one
MD5) is signed (encrypted with the private key). It is encoded with MD5) is signed (encrypted with the private key). It is encoded with
PKCS #1 block type 0 or type 1 as described in [PKCS1]. PKCS #1 block type 0 or type 1 as described in [PKCS1].
In DSS, the 20 bytes of the SHA hash are run directly through the In DSS, the 20 bytes of the SHA hash are run directly through the
Digital Signing Algorithm with no additional hashing. This produces Digital Signing Algorithm with no additional hashing. This produces
two values, r and s. The DSS signature is an opaque vector, as two values, r and s. The DSS signature is an opaque vector, as above,
above, the contents of which are the DER encoding of: the contents of which are the DER encoding of:
Dss-Sig-Value ::= SEQUENCE { Dss-Sig-Value ::= SEQUENCE {
r INTEGER, r INTEGER,
s INTEGER s INTEGER
} }
In stream cipher encryption, the plaintext is exclusive-ORed with an In stream cipher encryption, the plaintext is exclusive-ORed with an
identical amount of output generated from a cryptographically-secure identical amount of output generated from a cryptographically-secure
keyed pseudorandom number generator. keyed pseudorandom number generator.
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stream-ciphered struct { stream-ciphered struct {
uint8 field1; uint8 field1;
uint8 field2; uint8 field2;
digitally-signed opaque hash[20]; digitally-signed opaque hash[20];
} UserType; } UserType;
The contents of hash are used as input for the signing algorithm, The contents of hash are used as input for the signing algorithm,
then the entire structure is encrypted with a stream cipher. The then the entire structure is encrypted with a stream cipher. The
length of this structure, in bytes would be equal to 2 bytes for length of this structure, in bytes would be equal to 2 bytes for
field1 and field2, plus two bytes for the length of the signature, field1 and field2, plus two bytes for the length of the signature,
plus the length of the output of the signing algorithm. This is plus the length of the output of the signing algorithm. This is known
known due to the fact that the algorithm and key used for the due to the fact that the algorithm and key used for the signing are
signing are known prior to encoding or decoding this structure. known prior to encoding or decoding this structure.
4.8. Constants 4.8. Constants
Typed constants can be defined for purposes of specification by Typed constants can be defined for purposes of specification by
declaring a symbol of the desired type and assigning values to it. declaring a symbol of the desired type and assigning values to it.
Under-specified types (opaque, variable length vectors, and Under-specified types (opaque, variable length vectors, and
structures that contain opaque) cannot be assigned values. No fields structures that contain opaque) cannot be assigned values. No fields
of a multi-element structure or vector may be elided. of a multi-element structure or vector may be elided.
For example, For example,
struct { struct {
uint8 f1; uint8 f1;
uint8 f2; uint8 f2;
} Example1; } Example1;
Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */ Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
5. HMAC and the pseudorandom function 5. HMAC and the pseudorandom function
A number of operations in the TLS record and handshake layer A number of operations in the TLS record and handshake layer required
required a keyed MAC; this is a secure digest of some data protected a keyed MAC; this is a secure digest of some data protected by a
by a secret. Forging the MAC is infeasible without knowledge of the secret. Forging the MAC is infeasible without knowledge of the MAC
MAC secret. The construction we use for this operation is known as secret. The construction we use for this operation is known as HMAC,
HMAC, described in [HMAC]. described in [HMAC].
HMAC can be used with a variety of different hash algorithms. TLS HMAC can be used with a variety of different hash algorithms. TLS
uses it in the handshake with two different algorithms: MD5 and uses it in the handshake with two different algorithms: MD5 and SHA-
SHA-1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret, 1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret,
data). Additional hash algorithms can be defined by cipher suites data). Additional hash algorithms can be defined by cipher suites and
and used to protect record data, but MD5 and SHA-1 are hard coded used to protect record data, but MD5 and SHA-1 are hard coded into
into the description of the handshaking for this version of the the description of the handshaking for this version of the protocol.
protocol.
In addition, a construction is required to do expansion of secrets In addition, a construction is required to do expansion of secrets
into blocks of data for the purposes of key generation or into blocks of data for the purposes of key generation or validation.
validation. This pseudo-random function (PRF) takes as input a This pseudo-random function (PRF) takes as input a secret, a seed,
secret, a seed, and an identifying label and produces an output of and an identifying label and produces an output of arbitrary length.
arbitrary length.
In order to make the PRF as secure as possible, it uses two hash In order to make the PRF as secure as possible, it uses two hash
algorithms in a way which should guarantee its security if either algorithms in a way which should guarantee its security if either
algorithm remains secure. algorithm remains secure.
First, we define a data expansion function, P_hash(secret, data) First, we define a data expansion function, P_hash(secret, data)
which uses a single hash function to expand a secret and seed into which uses a single hash function to expand a secret and seed into an
an arbitrary quantity of output: arbitrary quantity of output:
P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) + P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
HMAC_hash(secret, A(2) + seed) + HMAC_hash(secret, A(2) + seed) +
HMAC_hash(secret, A(3) + seed) + ... HMAC_hash(secret, A(3) + seed) + ...
Where + indicates concatenation. Where + indicates concatenation.
A() is defined as: A() is defined as:
A(0) = seed A(0) = seed
A(i) = HMAC_hash(secret, A(i-1)) A(i) = HMAC_hash(secret, A(i-1))
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of the final iteration would then be discarded, leaving 64 bytes of of the final iteration would then be discarded, leaving 64 bytes of
output data. output data.
TLS's PRF is created by splitting the secret into two halves and TLS's PRF is created by splitting the secret into two halves and
using one half to generate data with P_MD5 and the other half to using one half to generate data with P_MD5 and the other half to
generate data with P_SHA-1, then exclusive-or'ing the outputs of generate data with P_SHA-1, then exclusive-or'ing the outputs of
these two expansion functions together. these two expansion functions together.
S1 and S2 are the two halves of the secret and each is the same S1 and S2 are the two halves of the secret and each is the same
length. S1 is taken from the first half of the secret, S2 from the length. S1 is taken from the first half of the secret, S2 from the
second half. Their length is created by rounding up the length of second half. Their length is created by rounding up the length of the
the overall secret divided by two; thus, if the original secret is overall secret divided by two; thus, if the original secret is an odd
an odd number of bytes long, the last byte of S1 will be the same as number of bytes long, the last byte of S1 will be the same as the
the first byte of S2. first byte of S2.
L_S = length in bytes of secret; L_S = length in bytes of secret;
L_S1 = L_S2 = ceil(L_S / 2); L_S1 = L_S2 = ceil(L_S / 2);
The secret is partitioned into two halves (with the possibility of The secret is partitioned into two halves (with the possibility of
one shared byte) as described above, S1 taking the first L_S1 bytes one shared byte) as described above, S1 taking the first L_S1 bytes
and S2 the last L_S2 bytes. and S2 the last L_S2 bytes.
The PRF is then defined as the result of mixing the two pseudorandom The PRF is then defined as the result of mixing the two pseudorandom
streams by exclusive-or'ing them together. streams by exclusive-or'ing them together.
PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
P_SHA-1(S2, label + seed); P_SHA-1(S2, label + seed);
The label is an ASCII string. It should be included in the exact The label is an ASCII string. It should be included in the exact form
form it is given without a length byte or trailing null character. it is given without a length byte or trailing null character. For
For example, the label "slithy toves" would be processed by hashing example, the label "slithy toves" would be processed by hashing the
the following bytes: following bytes:
73 6C 69 74 68 79 20 74 6F 76 65 73 73 6C 69 74 68 79 20 74 6F 76 65 73
Note that because MD5 produces 16 byte outputs and SHA-1 produces 20 Note that because MD5 produces 16 byte outputs and SHA-1 produces 20
byte outputs, the boundaries of their internal iterations will not byte outputs, the boundaries of their internal iterations will not be
be aligned; to generate a 80 byte output will involve P_MD5 being aligned; to generate a 80 byte output will involve P_MD5 being
iterated through A(5), while P_SHA-1 will only iterate through A(4). iterated through A(5), while P_SHA-1 will only iterate through A(4).
6. The TLS Record Protocol 6. The TLS Record Protocol
The TLS Record Protocol is a layered protocol. At each layer, The TLS Record Protocol is a layered protocol. At each layer,
messages may include fields for length, description, and content. messages may include fields for length, description, and content.
The Record Protocol takes messages to be transmitted, fragments the The Record Protocol takes messages to be transmitted, fragments the
data into manageable blocks, optionally compresses the data, applies data into manageable blocks, optionally compresses the data, applies
a MAC, encrypts, and transmits the result. Received data is a MAC, encrypts, and transmits the result. Received data is
decrypted, verified, decompressed, and reassembled, then delivered decrypted, verified, decompressed, and reassembled, then delivered to
to higher level clients. higher level clients.
Four record protocol clients are described in this document: the Four record protocol clients are described in this document: the
handshake protocol, the alert protocol, the change cipher spec handshake protocol, the alert protocol, the change cipher spec
protocol, and the application data protocol. In order to allow protocol, and the application data protocol. In order to allow
extension of the TLS protocol, additional record types can be extension of the TLS protocol, additional record types can be
supported by the record protocol. Any new record types should supported by the record protocol. Any new record types should
allocate type values immediately beyond the ContentType values for allocate type values immediately beyond the ContentType values for
the four record types described here (see Appendix A.2). If a TLS the four record types described here (see Appendix A.2). If a TLS
implementation receives a record type it does not understand, it implementation receives a record type it does not understand, it
should just ignore it. Any protocol designed for use over TLS must should just ignore it. Any protocol designed for use over TLS must be
be carefully designed to deal with all possible attacks against it. carefully designed to deal with all possible attacks against it.
Note that because the type and length of a record are not protected Note that because the type and length of a record are not protected
by encryption, care should be take to minimize the value of traffic by encryption, care should be take to minimize the value of traffic
analysis of these values. analysis of these values.
6.1. Connection states 6.1. Connection states
A TLS connection state is the operating environment of the TLS A TLS connection state is the operating environment of the TLS Record
Record Protocol. It specifies a compression algorithm, encryption Protocol. It specifies a compression algorithm, encryption algorithm,
algorithm, and MAC algorithm. In addition, the parameters for these and MAC algorithm. In addition, the parameters for these algorithms
algorithms are known: the MAC secret and the bulk encryption keys are known: the MAC secret and the bulk encryption keys and IVs for
and IVs for the connection in both the read and the write the connection in both the read and the write directions. Logically,
directions. Logically, there are always four connection states there are always four connection states outstanding: the current read
outstanding: the current read and write states, and the pending read and write states, and the pending read and write states. All records
and write states. All records are processed under the current read are processed under the current read and write states. The security
and write states. The security parameters for the pending states can parameters for the pending states can be set by the TLS Handshake
be set by the TLS Handshake Protocol, and the Handshake Protocol can Protocol, and the Handshake Protocol can selectively make either of
selectively make either of the pending states current, in which case the pending states current, in which case the appropriate current
the appropriate current state is disposed of and replaced with the state is disposed of and replaced with the pending state; the pending
pending state; the pending state is then reinitialized to an empty state is then reinitialized to an empty state. It is illegal to make
state. It is illegal to make a state which has not been initialized a state which has not been initialized with security parameters a
with security parameters a current state. The initial current state current state. The initial current state always specifies that no
always specifies that no encryption, compression, or MAC will be encryption, compression, or MAC will be used.
used.
The security parameters for a TLS Connection read and write state The security parameters for a TLS Connection read and write state are
are set by providing the following values: set by providing the following values:
connection end connection end
Whether this entity is considered the "client" or the "server" Whether this entity is considered the "client" or the "server" in
in this connection. this connection.
bulk encryption algorithm bulk encryption algorithm
An algorithm to be used for bulk encryption. This specification An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, how much of that key is includes the key size of this algorithm, how much of that key is
secret, whether it is a block or stream cipher, the block size secret, whether it is a block or stream cipher, the block size of
of the cipher (if appropriate), and whether it is considered an the cipher (if appropriate), and whether it is considered an
"export" cipher. "export" cipher.
MAC algorithm MAC algorithm
An algorithm to be used for message authentication. This An algorithm to be used for message authentication. This
specification includes the size of the hash which is returned by specification includes the size of the hash which is returned by
the MAC algorithm. the MAC algorithm.
compression algorithm compression algorithm
An algorithm to be used for data compression. This specification An algorithm to be used for data compression. This specification
must include all information the algorithm requires to do must include all information the algorithm requires to do
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The record layer will use the security parameters to generate the The record layer will use the security parameters to generate the
following six items: following six items:
client write MAC secret client write MAC secret
server write MAC secret server write MAC secret
client write key client write key
server write key server write key
client write IV (for block ciphers only) client write IV (for block ciphers only)
server write IV (for block ciphers only) server write IV (for block ciphers only)
The client write parameters are used by the server when receiving The client write parameters are used by the server when receiving and
and processing records and vice-versa. The algorithm used for processing records and vice-versa. The algorithm used for generating
generating these items from the security parameters is described in these items from the security parameters is described in section 6.3.
section 6.3.
Once the security parameters have been set and the keys have been Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them generated, the connection states can be instantiated by making them
the current states. These current states must be updated for each the current states. These current states must be updated for each
record processed. Each connection state includes the following record processed. Each connection state includes the following
elements: elements:
compression state compression state
The current state of the compression algorithm. The current state of the compression algorithm.
cipher state cipher state
The current state of the encryption algorithm. This will consist The current state of the encryption algorithm. This will consist
of the scheduled key for that connection. In addition, for block of the scheduled key for that connection. In addition, for block
ciphers running in CBC mode (the only mode specified for TLS), ciphers running in CBC mode (the only mode specified for TLS),
this will initially contain the IV for that connection state and this will initially contain the IV for that connection state and
be updated to contain the ciphertext of the last block encrypted be updated to contain the ciphertext of the last block encrypted
or decrypted as records are processed. For stream ciphers, this or decrypted as records are processed. For stream ciphers, this
will contain whatever the necessary state information is to will contain whatever the necessary state information is to allow
allow the stream to continue to encrypt or decrypt data. the stream to continue to encrypt or decrypt data.
MAC secret MAC secret
The MAC secret for this connection as generated above. The MAC secret for this connection as generated above.
sequence number sequence number
Each connection state contains a sequence number, which is Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence maintained separately for read and write states. The sequence
number must be set to zero whenever a connection state is made number must be set to zero whenever a connection state is made
the active state. Sequence numbers are of type uint64 and may the active state. Sequence numbers are of type uint64 and may not
not exceed 2^64-1. A sequence number is incremented after each exceed 2^64-1. A sequence number is incremented after each
record: specifically, the first record which is transmitted record: specifically, the first record which is transmitted under
under a particular connection state should use sequence number a particular connection state should use sequence number 0.
0.
6.2. Record layer 6.2. Record layer
The TLS Record Layer receives uninterpreted data from higher layers The TLS Record Layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size. in non-empty blocks of arbitrary size.
6.2.1. Fragmentation 6.2.1. Fragmentation
The record layer fragments information blocks into TLSPlaintext The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Client records carrying data in chunks of 2^14 bytes or less. Client message
message boundaries are not preserved in the record layer (i.e., boundaries are not preserved in the record layer (i.e., multiple
multiple client messages of the same ContentType may be coalesced client messages of the same ContentType may be coalesced into a
into a single TLSPlaintext record, or a single message may be single TLSPlaintext record, or a single message may be fragmented
fragmented across several records). across several records).
struct { struct {
uint8 major, minor; uint8 major, minor;
} ProtocolVersion; } ProtocolVersion;
enum { enum {
change_cipher_spec(20), alert(21), handshake(22), change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255) application_data(23), (255)
} ContentType; } ContentType;
struct { struct {
ContentType type; ContentType type;
ProtocolVersion version; ProtocolVersion version;
uint16 length; uint16 length;
opaque fragment[TLSPlaintext.length]; opaque fragment[TLSPlaintext.length];
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The application data. This data is transparent and treated as an The application data. This data is transparent and treated as an
independent block to be dealt with by the higher level protocol independent block to be dealt with by the higher level protocol
specified by the type field. specified by the type field.
Note: Data of different TLS Record layer content types may be Note: Data of different TLS Record layer content types may be
interleaved. Application data is generally of lower precedence interleaved. Application data is generally of lower precedence
for transmission than other content types. for transmission than other content types.
6.2.2. Record compression and decompression 6.2.2. Record compression and decompression
All records are compressed using the compression algorithm defined All records are compressed using the compression algorithm defined in
in the current session state. There is always an active compression the current session state. There is always an active compression
algorithm; however, initially it is defined as algorithm; however, initially it is defined as
CompressionMethod.null. The compression algorithm translates a CompressionMethod.null. The compression algorithm translates a
TLSPlaintext structure into a TLSCompressed structure. Compression TLSPlaintext structure into a TLSCompressed structure. Compression
functions are initialized with default state information whenever a functions are initialized with default state information whenever a
connection state is made active. connection state is made active.
Compression must be lossless and may not increase the content length Compression must be lossless and may not increase the content length
by more than 1024 bytes. If the decompression function encounters a by more than 1024 bytes. If the decompression function encounters a
TLSCompressed.fragment that would decompress to a length in excess TLSCompressed.fragment that would decompress to a length in excess of
of 2^14 bytes, it should report a fatal decompression failure error. 2^14 bytes, it should report a fatal decompression failure error.
struct { struct {
ContentType type; /* same as TLSPlaintext.type */ ContentType type; /* same as TLSPlaintext.type */
ProtocolVersion version;/* same as TLSPlaintext.version */ ProtocolVersion version;/* same as TLSPlaintext.version */
uint16 length; uint16 length;
opaque fragment[TLSCompressed.length]; opaque fragment[TLSCompressed.length];
} TLSCompressed; } TLSCompressed;
length length
The length (in bytes) of the following TLSCompressed.fragment. The length (in bytes) of the following TLSCompressed.fragment.
skipping to change at page 18, line 45 skipping to change at page 19, line 39
where "+" denotes concatenation. where "+" denotes concatenation.
seq_num seq_num
The sequence number for this record. The sequence number for this record.
hash hash
The hashing algorithm specified by The hashing algorithm specified by
SecurityParameters.mac_algorithm. SecurityParameters.mac_algorithm.
Note that the MAC is computed before encryption. The stream cipher Note that the MAC is computed before encryption. The stream cipher
encrypts the entire block, including the MAC. For stream ciphers encrypts the entire block, including the MAC. For stream ciphers that
that do not use a synchronization vector (such as RC4), the stream do not use a synchronization vector (such as RC4), the stream cipher
cipher state from the end of one record is simply used on the state from the end of one record is simply used on the subsequent
subsequent packet. If the CipherSuite is TLS_NULL_WITH_NULL_NULL, packet. If the CipherSuite is TLS_NULL_WITH_NULL_NULL, encryption
encryption consists of the identity operation (i.e., the data is not consists of the identity operation (i.e., the data is not encrypted
encrypted and the MAC size is zero implying that no MAC is used). and the MAC size is zero implying that no MAC is used).
TLSCiphertext.length is TLSCompressed.length plus TLSCiphertext.length is TLSCompressed.length plus
CipherSpec.hash_size. CipherSpec.hash_size.
6.2.3.2. CBC block cipher 6.2.3.2. CBC block cipher
For block ciphers (such as RC2 or DES), the encryption and MAC For block ciphers (such as RC2 or DES), the encryption and MAC
functions convert TLSCompressed.fragment structures to and from functions convert TLSCompressed.fragment structures to and from block
block TLSCiphertext.fragment structures. TLSCiphertext.fragment structures.
block-ciphered struct { block-ciphered struct {
opaque content[TLSCompressed.length]; opaque content[TLSCompressed.length];
opaque MAC[CipherSpec.hash_size]; opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length]; uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length; uint8 padding_length;
} GenericBlockCipher; } GenericBlockCipher;
The MAC is generated as described in Section 6.2.3.1. The MAC is generated as described in Section 6.2.3.1.
padding padding
Padding that is added to force the length of the plaintext to be Padding that is added to force the length of the plaintext to be
an integral multiple of the block cipher's block length. The an integral multiple of the block cipher's block length. The
padding may be any length up to 255 bytes long, as long as it padding may be any length up to 255 bytes long, as long as it
results in the TLSCiphertext.length being an integral multiple results in the TLSCiphertext.length being an integral multiple of
of the block length. Lengths longer than necessary might be the block length. Lengths longer than necessary might be
desirable to frustrate attacks on a protocol based on analysis desirable to frustrate attacks on a protocol based on analysis of
of the lengths of exchanged messages. Each uint8 in the padding the lengths of exchanged messages. Each uint8 in the padding data
data vector must be filled with the padding length value. vector must be filled with the padding length value.
padding_length padding_length
The padding length should be such that the total size of the The padding length should be such that the total size of the
GenericBlockCipher structure is a multiple of the cipher's block GenericBlockCipher structure is a multiple of the cipher's block
length. Legal values range from zero to 255, inclusive. This length. Legal values range from zero to 255, inclusive. This
length specifies the length of the padding field exclusive of length specifies the length of the padding field exclusive of the
the padding_length field itself. padding_length field itself.
The encrypted data length (TLSCiphertext.length) is one more than The encrypted data length (TLSCiphertext.length) is one more than the
the sum of TLSCompressed.length, CipherSpec.hash_size, and sum of TLSCompressed.length, CipherSpec.hash_size, and
padding_length. padding_length.
Example: If the block length is 8 bytes, the content length Example: If the block length is 8 bytes, the content length
(TLSCompressed.length) is 61 bytes, and the MAC length is 20 (TLSCompressed.length) is 61 bytes, and the MAC length is 20
bytes, the length before padding is 82 bytes. Thus, the bytes, the length before padding is 82 bytes. Thus, the
padding length modulo 8 must be equal to 6 in order to make padding length modulo 8 must be equal to 6 in order to make
the total length an even multiple of 8 bytes (the block the total length an even multiple of 8 bytes (the block
length). The padding length can be 6, 14, 22, and so on, length). The padding length can be 6, 14, 22, and so on,
through 254. If the padding length were the minimum necessary, through 254. If the padding length were the minimum necessary,
6, the padding would be 6 bytes, each containing the value 6. 6, the padding would be 6 bytes, each containing the value 6.
Thus, the last 8 octets of the GenericBlockCipher before block Thus, the last 8 octets of the GenericBlockCipher before block
encryption would be xx 06 06 06 06 06 06 06, where xx is the encryption would be xx 06 06 06 06 06 06 06, where xx is the
last octet of the MAC. last octet of the MAC.
Note: With block ciphers in CBC mode (Cipher Block Chaining) the Note: With block ciphers in CBC mode (Cipher Block Chaining) the
initialization vector (IV) for the first record is generated initialization vector (IV) for the first record is generated with
with the other keys and secrets when the security parameters are the other keys and secrets when the security parameters are set.
set. The IV for subsequent records is the last ciphertext block The IV for subsequent records is the last ciphertext block from
from the previous record. the previous record.
6.3. Key calculation 6.3. Key calculation
The Record Protocol requires an algorithm to generate keys, IVs, and The Record Protocol requires an algorithm to generate keys, IVs, and
MAC secrets from the security parameters provided by the handshake MAC secrets from the security parameters provided by the handshake
protocol. protocol.
The master secret is hashed into a sequence of secure bytes, which The master secret is hashed into a sequence of secure bytes, which
are assigned to the MAC secrets, keys, and non-export IVs required are assigned to the MAC secrets, keys, and non-export IVs required by
by the current connection state (see Appendix A.6). CipherSpecs the current connection state (see Appendix A.6). CipherSpecs require
require a client write MAC secret, a server write MAC secret, a a client write MAC secret, a server write MAC secret, a client write
client write key, a server write key, a client write IV, and a key, a server write key, a client write IV, and a server write IV,
server write IV, which are generated from the master secret in that which are generated from the master secret in that order. Unused
order. Unused values are empty. values are empty.
When generating keys and MAC secrets, the master secret is used as When generating keys and MAC secrets, the master secret is used as an
an entropy source, and the random values provide unencrypted salt entropy source, and the random values provide unencrypted salt
material and IVs for exportable ciphers. material and IVs for exportable ciphers.
To generate the key material, compute To generate the key material, compute
key_block = PRF(SecurityParameters.master_secret, key_block = PRF(SecurityParameters.master_secret,
"key expansion", "key expansion",
SecurityParameters.server_random + SecurityParameters.server_random +
SecurityParameters.client_random); SecurityParameters.client_random);
until enough output has been generated. Then the key_block is until enough output has been generated. Then the key_block is
partitioned as follows: partitioned as follows:
client_write_MAC_secret[SecurityParameters.hash_size] client_write_MAC_secret[SecurityParameters.hash_size]
server_write_MAC_secret[SecurityParameters.hash_size] server_write_MAC_secret[SecurityParameters.hash_size]
client_write_key[SecurityParameters.key_material] client_write_key[SecurityParameters.key_material_length]
server_write_key[SecurityParameters.key_material] server_write_key[SecurityParameters.key_material_length]
client_write_IV[SecurityParameters.IV_size] client_write_IV[SecurityParameters.IV_size]
server_write_IV[SecurityParameters.IV_size] server_write_IV[SecurityParameters.IV_size]
The client_write_IV and server_write_IV are only generated for The client_write_IV and server_write_IV are only generated for non-
non-export block ciphers. For exportable block ciphers, the export block ciphers. For exportable block ciphers, the
initialization vectors are generated later, as described below. Any initialization vectors are generated later, as described below. Any
extra key_block material is discarded. extra key_block material is discarded.
Implementation note: Implementation note:
The cipher spec which is defined in this document which requires The cipher spec which is defined in this document which requires
the most material is 3DES_EDE_CBC_SHA: it requires 2 x 24 byte the most material is 3DES_EDE_CBC_SHA: it requires 2 x 24 byte
keys, 2 x 20 byte MAC secrets, and 2 x 8 byte IVs, for a total keys, 2 x 20 byte MAC secrets, and 2 x 8 byte IVs, for a total of
of 104 bytes of key material. 104 bytes of key material.
Exportable encryption algorithms (for which CipherSpec.is_exportable Exportable encryption algorithms (for which CipherSpec.is_exportable
is true) require additional processing as follows to derive their is true) require additional processing as follows to derive their
final write keys: final write keys:
final_client_write_key = final_client_write_key =
PRF(SecurityParameters.client_write_key, PRF(SecurityParameters.client_write_key,
"client write key", "client write key",
SecurityParameters.client_random + SecurityParameters.client_random +
SecurityParameters.server_random); SecurityParameters.server_random);
final_server_write_key = final_server_write_key =
PRF(SecurityParameters.server_write_key, PRF(SecurityParameters.server_write_key,
"server write key", "server write key",
SecurityParameters.client_random + SecurityParameters.client_random +
SecurityParameters.server_random); SecurityParameters.server_random);
Exportable encryption algorithms derive their IVs solely from the Exportable encryption algorithms derive their IVs solely from the
random values from the hello messages: random values from the hello messages:
iv_block = PRF("", "IV block", SecurityParameters.client_random iv_block = PRF("", "IV block", SecurityParameters.client_random +
+
SecurityParameters.server_random); SecurityParameters.server_random);
The iv_block is partitioned into two initialization vectors as the The iv_block is partitioned into two initialization vectors as the
key_block was above: key_block was above:
client_write_IV[SecurityParameters.IV_size] client_write_IV[SecurityParameters.IV_size]
server_write_IV[SecurityParameters.IV_size] server_write_IV[SecurityParameters.IV_size]
Note that the PRF is used without a secret in this case: this just Note that the PRF is used without a secret in this case: this just
means that the secret has a length of zero bytes and contributes means that the secret has a length of zero bytes and contributes
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client_random + client_random +
server_random)[0..15] server_random)[0..15]
iv_block = PRF("", "IV block", client_random + iv_block = PRF("", "IV block", client_random +
server_random)[0..15] server_random)[0..15]
client_write_IV = iv_block[0..7] client_write_IV = iv_block[0..7]
server_write_IV = iv_block[8..15] server_write_IV = iv_block[8..15]
7. The TLS Handshake Protocol 7. The TLS Handshake Protocol
The TLS Handshake Protocol consists of a suite of three The TLS Handshake Protocol consists of a suite of three sub-protocols
sub-protocols which are used to allow peers to agree upon security which are used to allow peers to agree upon security parameters for
parameters for the record layer, authenticate themselves, the record layer, authenticate themselves, instantiate negotiated
instantiate negotiated security parameters, and report error security parameters, and report error conditions to each other.
conditions to each other.
The Handshake Protocol is responsible for negotiating a session, The Handshake Protocol is responsible for negotiating a session,
which consists of the following items: which consists of the following items:
session identifier session identifier
An arbitrary byte sequence chosen by the server to identify an An arbitrary byte sequence chosen by the server to identify an
active or resumable session state. active or resumable session state.
peer certificate peer certificate
X509v3[X509] certificate of the peer. This element of the state X509v3 [X509] certificate of the peer. This element of the state
may be null. may be null.
compression method compression method
The algorithm used to compress data prior to encryption. The algorithm used to compress data prior to encryption.
cipher spec cipher spec
Specifies the bulk data encryption algorithm (such as null, DES, Specifies the bulk data encryption algorithm (such as null, DES,
etc.) and a MAC algorithm (such as MD5 or SHA). It also defines etc.) and a MAC algorithm (such as MD5 or SHA). It also defines
cryptographic attributes such as the hash_size. (See Appendix cryptographic attributes such as the hash_size. (See Appendix A.6
A.6 for formal definition) for formal definition)
master secret master secret
48-byte secret shared between the client and server. 48-byte secret shared between the client and server.
is resumable is resumable
A flag indicating whether the session can be used to initiate A flag indicating whether the session can be used to initiate new
new connections. connections.
These items are then used to create security parameters for use by These items are then used to create security parameters for use by
the Record Layer when protecting application data. Many connections the Record Layer when protecting application data. Many connections
can be instantiated using the same session through the resumption can be instantiated using the same session through the resumption
feature of the TLS Handshake Protocol. feature of the TLS Handshake Protocol.
7.1. Change cipher spec protocol 7.1. Change cipher spec protocol
The change cipher spec protocol exists to signal transitions in The change cipher spec protocol exists to signal transitions in
ciphering strategies. The protocol consists of a single message, ciphering strategies. The protocol consists of a single message,
which is encrypted and compressed under the current (not the which is encrypted and compressed under the current (not the pending)
pending) connection state. The message consists of a single byte of connection state. The message consists of a single byte of value 1.
value 1.
struct { struct {
enum { change_cipher_spec(1), (255) } type; enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec; } ChangeCipherSpec;
The change cipher spec message is sent by both the client and server The change cipher spec message is sent by both the client and server
to notify the receiving party that subsequent records will be to notify the receiving party that subsequent records will be
protected under the newly negotiated CipherSpec and keys. Reception protected under the newly negotiated CipherSpec and keys. Reception
of this message causes the receiver to instruct the Record Layer to of this message causes the receiver to instruct the Record Layer to
immediately copy the read pending state into the read current state. immediately copy the read pending state into the read current state.
skipping to change at page 23, line 25 skipping to change at page 24, line 37
the record layer to make the write pending state the write active the record layer to make the write pending state the write active
state. (See section 6.1.) The change cipher spec message is sent state. (See section 6.1.) The change cipher spec message is sent
during the handshake after the security parameters have been agreed during the handshake after the security parameters have been agreed
upon, but before the verifying finished message is sent (see section upon, but before the verifying finished message is sent (see section
7.4.9). 7.4.9).
7.2. Alert protocol 7.2. Alert protocol
One of the content types supported by the TLS Record layer is the One of the content types supported by the TLS Record layer is the
alert type. Alert messages convey the severity of the message and a alert type. Alert messages convey the severity of the message and a
description of the alert. Alert messages with a level of fatal description of the alert. Alert messages with a level of fatal result
result in the immediate termination of the connection. In this case, in the immediate termination of the connection. In this case, other
other connections corresponding to the session may continue, but the connections corresponding to the session may continue, but the
session identifier must be invalidated, preventing the failed session identifier must be invalidated, preventing the failed session
session from being used to establish new connections. Like other from being used to establish new connections. Like other messages,
messages, alert messages are encrypted and compressed, as specified alert messages are encrypted and compressed, as specified by the
by the current connection state. current connection state.
enum { warning(1), fatal(2), (255) } AlertLevel; enum { warning(1), fatal(2), (255) } AlertLevel;
enum { enum {
close_notify(0), close_notify(0),
unexpected_message(10), unexpected_message(10),
bad_record_mac(20), bad_record_mac(20),
decryption_failed(21), decryption_failed(21),
record_overflow(22), record_overflow(22),
decompression_failure(30), decompression_failure(30),
skipping to change at page 24, line 16 skipping to change at page 25, line 32
(255) (255)
} AlertDescription; } AlertDescription;
struct { struct {
AlertLevel level; AlertLevel level;
AlertDescription description; AlertDescription description;
} Alert; } Alert;
7.2.1. Closure alerts 7.2.1. Closure alerts
The client and the server must share knowledge that the connection The client and the server must share knowledge that the connection is
is ending in order to avoid a truncation attack. Either party may ending in order to avoid a truncation attack. Either party may
initiate the exchange of closing messages. initiate the exchange of closing messages.
close_notify close_notify
This message notifies the recipient that the sender will not This message notifies the recipient that the sender will not send
send any more messages on this connection. The session becomes any more messages on this connection. The session becomes
unresumable if any connection is terminated without proper unresumable if any connection is terminated without proper
close_notify messages with level equal to warning. close_notify messages with level equal to warning.
Either party may initiate a close by sending a close_notify alert. Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored. Any data received after a closure alert is ignored.
Each party is required to send a close_notify alert before closing Each party is required to send a close_notify alert before closing
the write side of the connection. It is required that the other the write side of the connection. It is required that the other party
party respond with a close_notify alert of its own and close down respond with a close_notify alert of its own and close down the
the connection immediately, discarding any pending writes. It is not connection immediately, discarding any pending writes. It is not
required for the initiator of the close to wait for the responding required for the initiator of the close to wait for the responding
close_notify alert before closing the read side of the connection. close_notify alert before closing the read side of the connection.
If the application protocol using TLS provides that any data may be If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is carried over the underlying transport after the TLS connection is
closed, the TLS implementation must receive the responding closed, the TLS implementation must receive the responding
close_notify alert before indicating to the application layer that close_notify alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying transfer any additional data, but will only close the underlying
transport connection, then the implementation may choose to close transport connection, then the implementation may choose to close the
the transport without waiting for the responding close_notify. No transport without waiting for the responding close_notify. No part of
part of this standard should be taken to dictate the manner in which this standard should be taken to dictate the manner in which a usage
a usage profile for TLS manages its data transport, including when profile for TLS manages its data transport, including when
connections are opened or closed. connections are opened or closed.
NB: It is assumed that closing a connection reliably delivers NB: It is assumed that closing a connection reliably delivers
pending data before destroying the transport. pending data before destroying the transport.
7.2.2. Error alerts 7.2.2. Error alerts
Error handling in the TLS Handshake protocol is very simple. When an Error handling in the TLS Handshake protocol is very simple. When an
error is detected, the detecting party sends a message to the other error is detected, the detecting party sends a message to the other
party. Upon transmission or receipt of an fatal alert message, both party. Upon transmission or receipt of an fatal alert message, both
parties immediately close the connection. Servers and clients are parties immediately close the connection. Servers and clients are
required to forget any session-identifiers, keys, and secrets required to forget any session-identifiers, keys, and secrets
associated with a failed connection. The following error alerts are associated with a failed connection. The following error alerts are
defined: defined:
unexpected_message unexpected_message
An inappropriate message was received. This alert is always An inappropriate message was received. This alert is always fatal
fatal and should never be observed in communication between and should never be observed in communication between proper
proper implementations. implementations.
bad_record_mac bad_record_mac
This alert is returned if a record is received with an incorrect This alert is returned if a record is received with an incorrect
MAC. This message is always fatal. MAC. This message is always fatal.
decryption_failed decryption_failed
A TLSCiphertext decrypted in an invalid way: either it wasn`t an A TLSCiphertext decrypted in an invalid way: either it wasn`t an
even multiple of the block length or its padding values, when even multiple of the block length or its padding values, when
checked, weren`t correct. This message is always fatal. checked, weren`t correct. This message is always fatal.
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A TLSCiphertext record was received which had a length more than A TLSCiphertext record was received which had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed record 2^14+2048 bytes, or a record decrypted to a TLSCompressed record
with more than 2^14+1024 bytes. This message is always fatal. with more than 2^14+1024 bytes. This message is always fatal.
decompression_failure decompression_failure
The decompression function received improper input (e.g. data The decompression function received improper input (e.g. data
that would expand to excessive length). This message is always that would expand to excessive length). This message is always
fatal. fatal.
handshake_failure handshake_failure
Reception of a handshake_failure alert message indicates that Reception of a handshake_failure alert message indicates that the
the sender was unable to negotiate an acceptable set of security sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error. parameters given the options available. This is a fatal error.
bad_certificate bad_certificate
A certificate was corrupt, contained signatures that did not A certificate was corrupt, contained signatures that did not
verify correctly, etc. verify correctly, etc.
unsupported_certificate unsupported_certificate
A certificate was of an unsupported type. A certificate was of an unsupported type.
certificate_revoked certificate_revoked
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certificate_unknown certificate_unknown
Some other (unspecified) issue arose in processing the Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable. certificate, rendering it unacceptable.
illegal_parameter illegal_parameter
A field in the handshake was out of range or inconsistent with A field in the handshake was out of range or inconsistent with
other fields. This is always fatal. other fields. This is always fatal.
unknown_ca unknown_ca
A valid certificate chain or partial chain was received, but the A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could certificate was not accepted because the CA certificate could not
not be located or couldn`t be matched with a known, trusted CA. be located or couldn`t be matched with a known, trusted CA. This
This message is always fatal. message is always fatal.
access_denied access_denied
A valid certificate was received, but when access control was A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. applied, the sender decided not to proceed with negotiation.
This message is always fatal. This message is always fatal.
decode_error decode_error
A message could not be decoded because some field was out of the A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This specified range or the length of the message was incorrect. This
message is always fatal. message is always fatal.
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protocol_version protocol_version
The protocol version the client has attempted to negotiate is The protocol version the client has attempted to negotiate is
recognized, but not supported. (For example, old protocol recognized, but not supported. (For example, old protocol
versions might be avoided for security reasons). This message is versions might be avoided for security reasons). This message is
always fatal. always fatal.
insufficient_security insufficient_security
Returned instead of handshake_failure when a negotiation has Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more failed specifically because the server requires ciphers more
secure than those supported by the client. This message is secure than those supported by the client. This message is always
always fatal. fatal.
internal_error internal_error
An internal error unrelated to the peer or the correctness of An internal error unrelated to the peer or the correctness of the
the protocol makes it impossible to continue (such as a memory protocol makes it impossible to continue (such as a memory
allocation failure). This message is always fatal. allocation failure). This message is always fatal.
user_canceled user_canceled
This handshake is being canceled for some reason unrelated to a This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be followed close_notify is more appropriate. This alert should be followed
by a close_notify. This message is generally a warning. by a close_notify. This message is generally a warning.
no_renegotiation no_renegotiation
Sent by the client in response to a hello request or by the Sent by the client in response to a hello request or by the
server in response to a client hello after initial handshaking. server in response to a client hello after initial handshaking.
Either of these would normally lead to renegotiation; when that Either of these would normally lead to renegotiation; when that
is not appropriate, the recipient should respond with this is not appropriate, the recipient should respond with this alert;
alert; at that point, the original requester can decide whether at that point, the original requester can decide whether to
to proceed with the connection. One case where this would be proceed with the connection. One case where this would be
appropriate would be where a server has spawned a process to appropriate would be where a server has spawned a process to
satisfy a request; the process might receive security parameters satisfy a request; the process might receive security parameters
(key length, authentication, etc.) at startup and it might be (key length, authentication, etc.) at startup and it might be
difficult to communicate changes to these parameters after that difficult to communicate changes to these parameters after that
point. This message is always a warning. point. This message is always a warning.
For all errors where an alert level is not explicitly specified, the For all errors where an alert level is not explicitly specified, the
sending party may determine at its discretion whether this is a sending party may determine at its discretion whether this is a fatal
fatal error or not; if an alert with a level of warning is received, error or not; if an alert with a level of warning is received, the
the receiving party may decide at its discretion whether to treat receiving party may decide at its discretion whether to treat this as
this as a fatal error or not. However, all messages which are a fatal error or not. However, all messages which are transmitted
transmitted with a level of fatal must be treated as fatal messages. with a level of fatal must be treated as fatal messages.
7.3. Handshake Protocol overview 7.3. Handshake Protocol overview
The cryptographic parameters of the session state are produced by The cryptographic parameters of the session state are produced by the
the TLS Handshake Protocol, which operates on top of the TLS Record TLS Handshake Protocol, which operates on top of the TLS Record
Layer. When a TLS client and server first start communicating, they Layer. When a TLS client and server first start communicating, they
agree on a protocol version, select cryptographic algorithms, agree on a protocol version, select cryptographic algorithms,
optionally authenticate each other, and use public-key encryption optionally authenticate each other, and use public-key encryption
techniques to generate shared secrets. techniques to generate shared secrets.
The TLS Handshake Protocol involves the following steps: The TLS Handshake Protocol involves the following steps:
- Exchange hello messages to agree on algorithms, exchange random - Exchange hello messages to agree on algorithms, exchange random
values, and check for session resumption. values, and check for session resumption.
skipping to change at page 28, line 12 skipping to change at page 29, line 44
calculated the same security parameters and that the handshake calculated the same security parameters and that the handshake
occurred without tampering by an attacker. occurred without tampering by an attacker.
Note that higher layers should not be overly reliant on TLS always Note that higher layers should not be overly reliant on TLS always
negotiating the strongest possible connection between two peers: negotiating the strongest possible connection between two peers:
there are a number of ways a man in the middle attacker can attempt there are a number of ways a man in the middle attacker can attempt
to make two entities drop down to the least secure method they to make two entities drop down to the least secure method they
support. The protocol has been designed to minimize this risk, but support. The protocol has been designed to minimize this risk, but
there are still attacks available: for example, an attacker could there are still attacks available: for example, an attacker could
block access to the port a secure service runs on, or attempt to get block access to the port a secure service runs on, or attempt to get
the peers to negotiate an unauthenticated connection. The the peers to negotiate an unauthenticated connection. The fundamental
fundamental rule is that higher levels must be cognizant of what rule is that higher levels must be cognizant of what their security
their security requirements are and never transmit information over requirements are and never transmit information over a channel less
a channel less secure than what they require. The TLS protocol is secure than what they require. The TLS protocol is secure, in that
secure, in that any cipher suite offers its promised level of any cipher suite offers its promised level of security: if you
security: if you negotiate 3DES with a 1024 bit RSA key exchange negotiate 3DES with a 1024 bit RSA key exchange with a host whose
with a host whose certificate you have verified, you can expect to certificate you have verified, you can expect to be that secure.
be that secure. However, you should never send data over a link
encrypted with 40 bit security unless you feel that data is worth no However, you should never send data over a link encrypted with 40 bit
more than the effort required to break that encryption. security unless you feel that data is worth no more than the effort
required to break that encryption.
These goals are achieved by the handshake protocol, which can be These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a client hello message to summarized as follows: The client sends a client hello message to
which the server must respond with a server hello message, or else a which the server must respond with a server hello message, or else a
fatal error will occur and the connection will fail. The client fatal error will occur and the connection will fail. The client hello
hello and server hello are used to establish security enhancement and server hello are used to establish security enhancement
capabilities between client and server. The client hello and server capabilities between client and server. The client hello and server
hello establish the following attributes: Protocol Version, Session hello establish the following attributes: Protocol Version, Session
ID, Cipher Suite, and Compression Method. Additionally, two random ID, Cipher Suite, and Compression Method. Additionally, two random
values are generated and exchanged: ClientHello.random and values are generated and exchanged: ClientHello.random and
ServerHello.random. ServerHello.random.
The actual key exchange uses up to four messages: the server The actual key exchange uses up to four messages: the server
certificate, the server key exchange, the client certificate, and certificate, the server key exchange, the client certificate, and the
the client key exchange. New key exchange methods can be created by client key exchange. New key exchange methods can be created by
specifying a format for these messages and defining the use of the specifying a format for these messages and defining the use of the
messages to allow the client and server to agree upon a shared messages to allow the client and server to agree upon a shared
secret. This secret should be quite long; currently defined key secret. This secret should be quite long; currently defined key
exchange methods exchange secrets which range from 48 to 128 bytes exchange methods exchange secrets which range from 48 to 128 bytes in
in length. length.
Following the hello messages, the server will send its certificate, Following the hello messages, the server will send its certificate,
if it is to be authenticated. Additionally, a server key exchange if it is to be authenticated. Additionally, a server key exchange
message may be sent, if it is required (e.g. if their server has no message may be sent, if it is required (e.g. if their server has no
certificate, or if its certificate is for signing only). If the certificate, or if its certificate is for signing only). If the
server is authenticated, it may request a certificate from the server is authenticated, it may request a certificate from the
client, if that is appropriate to the cipher suite selected. Now the client, if that is appropriate to the cipher suite selected. Now the
server will send the server hello done message, indicating that the server will send the server hello done message, indicating that the
hello-message phase of the handshake is complete. The server will hello-message phase of the handshake is complete. The server will
then wait for a client response. If the server has sent a then wait for a client response. If the server has sent a certificate
certificate request message, the client must send the certificate request message, the client must send the certificate message. The
message. The client key exchange message is now sent, and the client key exchange message is now sent, and the content of that
content of that message will depend on the public key algorithm message will depend on the public key algorithm selected between the
selected between the client hello and the server hello. If the client hello and the server hello. If the client has sent a
client has sent a certificate with signing ability, a certificate with signing ability, a digitally-signed certificate
digitally-signed certificate verify message is sent to explicitly verify message is sent to explicitly verify the certificate.
verify the certificate.
At this point, a change cipher spec message is sent by the client, At this point, a change cipher spec message is sent by the client,
and the client copies the pending Cipher Spec into the current and the client copies the pending Cipher Spec into the current Cipher
Cipher Spec. The client then immediately sends the finished message Spec. The client then immediately sends the finished message under
under the new algorithms, keys, and secrets. In response, the server the new algorithms, keys, and secrets. In response, the server will
will send its own change cipher spec message, transfer the pending send its own change cipher spec message, transfer the pending to the
to the current Cipher Spec, and send its finished message under the current Cipher Spec, and send its finished message under the new
new Cipher Spec. At this point, the handshake is complete and the Cipher Spec. At this point, the handshake is complete and the client
client and server may begin to exchange application layer data. (See and server may begin to exchange application layer data. (See flow
flow chart below.) chart below.)
Client Server Client Server
ClientHello --------> ClientHello -------->
ServerHello ServerHello
Certificate* Certificate*
ServerKeyExchange* ServerKeyExchange*
CertificateRequest* CertificateRequest*
<-------- ServerHelloDone <-------- ServerHelloDone
Certificate* Certificate*
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Finished --------> Finished -------->
[ChangeCipherSpec] [ChangeCipherSpec]
<-------- Finished <-------- Finished
Application Data <-------> Application Data Application Data <-------> Application Data
Fig. 1 - Message flow for a full handshake Fig. 1 - Message flow for a full handshake
* Indicates optional or situation-dependent messages that are not * Indicates optional or situation-dependent messages that are not
always sent. always sent.
Note: To help avoid pipeline stalls, ChangeCipherSpec is an Note: To help avoid pipeline stalls, ChangeCipherSpec is an
independent TLS Protocol content type, and is not actually a TLS independent TLS Protocol content type, and is not actually a TLS
handshake message. handshake message.
When the client and server decide to resume a previous session or When the client and server decide to resume a previous session or
duplicate an existing session (instead of negotiating new security duplicate an existing session (instead of negotiating new security
parameters) the message flow is as follows: parameters) the message flow is as follows:
The client sends a ClientHello using the Session ID of the session The client sends a ClientHello using the Session ID of the session to
to be resumed. The server then checks its session cache for a match. be resumed. The server then checks its session cache for a match. If
If a match is found, and the server is willing to re-establish the a match is found, and the server is willing to re-establish the
connection under the specified session state, it will send a connection under the specified session state, it will send a
ServerHello with the same Session ID value. At this point, both ServerHello with the same Session ID value. At this point, both
client and server must send change cipher spec messages and proceed client and server must send change cipher spec messages and proceed
directly to finished messages. Once the re-establishment is directly to finished messages. Once the re-establishment is complete,
complete, the client and server may begin to exchange application the client and server may begin to exchange application layer data.
layer data. (See flow chart below.) If a Session ID match is not (See flow chart below.) If a Session ID match is not found, the
found, the server generates a new session ID and the TLS client and server generates a new session ID and the TLS client and server
server perform a full handshake. perform a full handshake.
Client Server Client Server
ClientHello --------> ClientHello -------->
ServerHello ServerHello
[ChangeCipherSpec] [ChangeCipherSpec]
<-------- Finished <-------- Finished
[ChangeCipherSpec] [ChangeCipherSpec]
Finished --------> Finished -------->
Application Data <-------> Application Data Application Data <-------> Application Data
Fig. 2 - Message flow for an abbreviated handshake Fig. 2 - Message flow for an abbreviated handshake
The contents and significance of each message will be presented in The contents and significance of each message will be presented in
detail in the following sections. detail in the following sections.
7.4. Handshake protocol 7.4. Handshake protocol
The TLS Handshake Protocol is one of the defined higher level The TLS Handshake Protocol is one of the defined higher level clients
clients of the TLS Record Protocol. This protocol is used to of the TLS Record Protocol. This protocol is used to negotiate the
negotiate the secure attributes of a session. Handshake messages are secure attributes of a session. Handshake messages are supplied to
supplied to the TLS Record Layer, where they are encapsulated within the TLS Record Layer, where they are encapsulated within one or more
one or more TLSPlaintext structures, which are processed and TLSPlaintext structures, which are processed and transmitted as
transmitted as specified by the current active session state. specified by the current active session state.
enum { enum {
hello_request(0), client_hello(1), server_hello(2), hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12), certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14), certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16), certificate_verify(15), client_key_exchange(16),
finished(20), (255) finished(20), (255)
} HandshakeType; } HandshakeType;
struct { struct {
skipping to change at page 31, line 5 skipping to change at page 33, line 5
case certificate: Certificate; case certificate: Certificate;
case server_key_exchange: ServerKeyExchange; case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest; case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone; case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify; case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange; case client_key_exchange: ClientKeyExchange;
case finished: Finished; case finished: Finished;
} body; } body;
} Handshake; } Handshake;
The handshake protocol messages are presented below in the order The handshake protocol messages are presented below in the order they
they must be sent; sending handshake messages in an unexpected order must be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be results in a fatal error. Unneeded handshake messages can be omitted,
omitted, however. Note one exception to the ordering: the however. Note one exception to the ordering: the Certificate message
Certificate message is used twice in the handshake (from server to is used twice in the handshake (from server to client, then from
client, then from client to server), but described only in its first client to server), but described only in its first position. The one
position. The one message which is not bound by these ordering rules message which is not bound by these ordering rules in the Hello
in the Hello Request message, which can be sent at any time, but Request message, which can be sent at any time, but which should be
which should be ignored by the client if it arrives in the middle of ignored by the client if it arrives in the middle of a handshake.
a handshake.
7.4.1. Hello messages 7.4.1. Hello messages
The hello phase messages are used to exchange security enhancement The hello phase messages are used to exchange security enhancement
capabilities between the client and server. When a new session capabilities between the client and server. When a new session
begins, the Record Layer's connection state encryption, hash, and begins, the Record Layer's connection state encryption, hash, and
compression algorithms are initialized to null. The current compression algorithms are initialized to null. The current
connection state is used for renegotiation messages. connection state is used for renegotiation messages.
7.4.1.1. Hello request 7.4.1.1. Hello request
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than a few records are received from the client. If the server than a few records are received from the client. If the server
sends a hello request but does not receive a client hello in sends a hello request but does not receive a client hello in
response, it may close the connection with a fatal alert. response, it may close the connection with a fatal alert.
After sending a hello request, servers should not repeat the request After sending a hello request, servers should not repeat the request
until the subsequent handshake negotiation is complete. until the subsequent handshake negotiation is complete.
Structure of this message: Structure of this message:
struct { } HelloRequest; struct { } HelloRequest;
Note: This message should never be included in the message hashes Note: This message should never be included in the message hashes which
which are maintained throughout the handshake and used in the are maintained throughout the handshake and used in the finished
finished messages and the certificate verify message. messages and the certificate verify message.
7.4.1.2. Client hello 7.4.1.2. Client hello
When this message will be sent: When this message will be sent:
When a client first connects to a server it is required to send When a client first connects to a server it is required to send
the client hello as its first message. The client can also send the client hello as its first message. The client can also send a
a client hello in response to a hello request or on its own client hello in response to a hello request or on its own
initiative in order to renegotiate the security parameters in an initiative in order to renegotiate the security parameters in an
existing connection. existing connection.
Structure of this message: Structure of this message:
The client hello message includes a random structure, which is The client hello message includes a random structure, which is
used later in the protocol. used later in the protocol.
struct { struct {
uint32 gmt_unix_time; uint32 gmt_unix_time;
opaque random_bytes[28]; opaque random_bytes[28];
} Random; } Random;
gmt_unix_time gmt_unix_time
The current time and date in standard UNIX 32-bit format The current time and date in standard UNIX 32-bit format (seconds
(seconds since the midnight starting Jan 1, 1970, GMT) according since the midnight starting Jan 1, 1970, GMT) according to the
to the sender's internal clock. Clocks are not required to be sender's internal clock. Clocks are not required to be set
set correctly by the basic TLS Protocol; higher level or correctly by the basic TLS Protocol; higher level or application
application protocols may define additional requirements. protocols may define additional requirements.
random_bytes random_bytes
28 bytes generated by a secure random number generator. 28 bytes generated by a secure random number generator.
The client hello message includes a variable length session The client hello message includes a variable length session
identifier. If not empty, the value identifies a session between the identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes same client and server whose security parameters the client wishes to
to reuse. The session identifier may be from an earlier connection, reuse. The session identifier may be from an earlier connection, this
this connection, or another currently active connection. The second connection, or another currently active connection. The second option
option is useful if the client only wishes to update the random is useful if the client only wishes to update the random structures
structures and derived values of a connection, while the third and derived values of a connection, while the third option makes it
option makes it possible to establish several independent secure possible to establish several independent secure connections without
connections without repeating the full handshake protocol. These repeating the full handshake protocol. These independent connections
independent connections may occur sequentially or simultaneously; a may occur sequentially or simultaneously; a SessionID becomes valid
SessionID becomes valid when the handshake negotiating it completes when the handshake negotiating it completes with the exchange of
with the exchange of Finished messages and persists until removed Finished messages and persists until removed due to aging or because
due to aging or because a fatal error was encountered on a a fatal error was encountered on a connection associated with the
connection associated with the session. The actual contents of the session. The actual contents of the SessionID are defined by the
SessionID are defined by the server. server.
opaque SessionID<0..32>; opaque SessionID<0..32>;
Warning: Warning:
Because the SessionID is transmitted without encryption or Because the SessionID is transmitted without encryption or
immediate MAC protection, servers must not place confidential immediate MAC protection, servers must not place confidential
information in session identifiers or let the contents of fake information in session identifiers or let the contents of fake
session identifiers cause any breach of security. (Note that the session identifiers cause any breach of security. (Note that the
content of the handshake as a whole, including the SessionID, is content of the handshake as a whole, including the SessionID, is
protected by the Finished messages exchanged at the end of the protected by the Finished messages exchanged at the end of the
handshake.) handshake.)
The CipherSuite list, passed from the client to the server in the The CipherSuite list, passed from the client to the server in the
client hello message, contains the combinations of cryptographic client hello message, contains the combinations of cryptographic
algorithms supported by the client in order of the client's algorithms supported by the client in order of the client's
preference (favorite choice first). Each CipherSuite defines a key preference (favorite choice first). Each CipherSuite defines a key
exchange algorithm, a bulk encryption algorithm (including secret exchange algorithm, a bulk encryption algorithm (including secret key
key length) and a MAC algorithm. The server will select a cipher length) and a MAC algorithm. The server will select a cipher suite
suite or, if no acceptable choices are presented, return a handshake or, if no acceptable choices are presented, return a handshake
failure alert and close the connection. failure alert and close the connection.
uint8 CipherSuite[2]; /* Cryptographic suite selector */ uint8 CipherSuite[2]; /* Cryptographic suite selector */
The client hello includes a list of compression algorithms supported The client hello includes a list of compression algorithms supported
by the client, ordered according to the client's preference. by the client, ordered according to the client's preference.
enum { null(0), (255) } CompressionMethod; enum { null(0), (255) } CompressionMethod;
struct { struct {
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The version of the TLS protocol by which the client wishes to The version of the TLS protocol by which the client wishes to
communicate during this session. This should be the latest communicate during this session. This should be the latest
(highest valued) version supported by the client. For this (highest valued) version supported by the client. For this
version of the specification, the version will be 3.1 (See version of the specification, the version will be 3.1 (See
Appendix E for details about backward compatibility). Appendix E for details about backward compatibility).
random random
A client-generated random structure. A client-generated random structure.
session_id session_id
The ID of a session the client wishes to use for this The ID of a session the client wishes to use for this connection.
connection. This field should be empty if no session_id is This field should be empty if no session_id is available or the
available or the client wishes to generate new security client wishes to generate new security parameters.
parameters.
cipher_suites cipher_suites
This is a list of the cryptographic options supported by the This is a list of the cryptographic options supported by the
client, with the client's first preference first. If the client, with the client's first preference first. If the
session_id field is not empty (implying a session resumption session_id field is not empty (implying a session resumption
request) this vector must include at least the cipher_suite from request) this vector must include at least the cipher_suite from
that session. Values are defined in Appendix A.5. that session. Values are defined in Appendix A.5.
compression_methods compression_methods
This is a list of the compression methods supported by the This is a list of the compression methods supported by the
client, sorted by client preference. If the session_id field is client, sorted by client preference. If the session_id field is
not empty (implying a session resumption request) it must not empty (implying a session resumption request) it must include
include the compression_method from that session. This vector the compression_method from that session. This vector must
must contain, and all implementations must support, contain, and all implementations must support,
CompressionMethod.null. Thus, a client and server will always be CompressionMethod.null. Thus, a client and server will always be
able to agree on a compression method. able to agree on a compression method.
After sending the client hello message, the client waits for a After sending the client hello message, the client waits for a server
server hello message. Any other handshake message returned by the hello message. Any other handshake message returned by the server
server except for a hello request is treated as a fatal error. except for a hello request is treated as a fatal error.
Forward compatibility note: Forward compatibility note:
In the interests of forward compatibility, it is permitted for a In the interests of forward compatibility, it is permitted for a
client hello message to include extra data after the compression client hello message to include extra data after the compression
methods. This data must be included in the handshake hashes, but methods. This data must be included in the handshake hashes, but
must otherwise be ignored. This is the only handshake message must otherwise be ignored. This is the only handshake message for
for which this is legal; for all other messages, the amount of which this is legal; for all other messages, the amount of data
data in the message must match the description of the message in the message must match the description of the message
precisely. precisely.
7.4.1.3. Server hello 7.4.1.3. Server hello
When this message will be sent: When this message will be sent:
The server will send this message in response to a client hello The server will send this message in response to a client hello
message when it was able to find an acceptable set of message when it was able to find an acceptable set of algorithms.
algorithms. If it cannot find such a match, it will respond with If it cannot find such a match, it will respond with a handshake
a handshake failure alert. failure alert.
Structure of this message: Structure of this message:
struct { struct {
ProtocolVersion server_version; ProtocolVersion server_version;
Random random; Random random;
SessionID session_id; SessionID session_id;
CipherSuite cipher_suite; CipherSuite cipher_suite;
CompressionMethod compression_method; CompressionMethod compression_method;
} ServerHello; } ServerHello;
server_version server_version
This field will contain the lower of that suggested by the This field will contain the lower of that suggested by the client
client in the client hello and the highest supported by the in the client hello and the highest supported by the server. For
server. For this version of the specification, the version is this version of the specification, the version is 3.1 (See
3.1 (See Appendix E for details about backward compatibility). Appendix E for details about backward compatibility).
random random
This structure is generated by the server and must be different This structure is generated by the server and must be different
from (and independent of) ClientHello.random. from (and independent of) ClientHello.random.
session_id session_id
This is the identity of the session corresponding to this This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection found and the server is willing to establish the new connection
using the specified session state, the server will respond with using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed resumed session and dictates that the parties must proceed
directly to the finished messages. Otherwise this field will directly to the finished messages. Otherwise this field will
contain a different value identifying the new session. The contain a different value identifying the new session. The server
server may return an empty session_id to indicate that the may return an empty session_id to indicate that the session will
session will not be cached and therefore cannot be resumed. If a not be cached and therefore cannot be resumed. If a session is
session is resumed, it must be resumed using the same cipher resumed, it must be resumed using the same cipher suite it was
suite it was originally negotiated with. originally negotiated with.
cipher_suite cipher_suite
The single cipher suite selected by the server from the list in The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions this field is ClientHello.cipher_suites. For resumed sessions this field is the
the value from the state of the session being resumed. value from the state of the session being resumed.
compression_method compression_method
The single compression algorithm selected by the server from the The single compression algorithm selected by the server from the
list in ClientHello.compression_methods. For resumed sessions list in ClientHello.compression_methods. For resumed sessions
this field is the value from the resumed session state. this field is the value from the resumed session state.
7.4.2. Server certificate 7.4.2. Server certificate
When this message will be sent: When this message will be sent:
The server must send a certificate whenever the agreed-upon key The server must send a certificate whenever the agreed-upon key
exchange method is not an anonymous one. This message will exchange method is not an anonymous one. This message will always
always immediately follow the server hello message. immediately follow the server hello message.
Meaning of this message: Meaning of this message:
The certificate type must be appropriate for the selected cipher The certificate type must be appropriate for the selected cipher
suite's key exchange algorithm, and is generally an X.509v3 suite's key exchange algorithm, and is generally an X.509v3
certificate. It must contain a key which matches the key certificate. It must contain a key which matches the key exchange
exchange method, as follows. Unless otherwise specified, the method, as follows. Unless otherwise specified, the signing
signing algorithm for the certificate must be the same as the algorithm for the certificate must be the same as the algorithm
algorithm for the certificate key. Unless otherwise specified, for the certificate key. Unless otherwise specified, the public
the public key may be of any length. key may be of any length.
Key Exchange Algorithm Certificate Key Type Key Exchange Algorithm Certificate Key Type
RSA RSA public key; the certificate must RSA RSA public key; the certificate must
allow the key to be used for encryption. allow the key to be used for encryption.
RSA_EXPORT RSA public key of length greater than RSA_EXPORT RSA public key of length greater than
512 bits which can be used for signing, 512 bits which can be used for signing,
or a key of 512 bits or shorter which or a key of 512 bits or shorter which
can be used for either encryption or can be used for either encryption or
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DH_DSS Diffie-Hellman key. The algorithm used DH_DSS Diffie-Hellman key. The algorithm used
to sign the certificate should be DSS. to sign the certificate should be DSS.
DH_RSA Diffie-Hellman key. The algorithm used DH_RSA Diffie-Hellman key. The algorithm used
to sign the certificate should be RSA. to sign the certificate should be RSA.
All certificate profiles, key and cryptographic formats are defined All certificate profiles, key and cryptographic formats are defined
by the IETF PKIX working group [PKIX]. When a key usage extension is by the IETF PKIX working group [PKIX]. When a key usage extension is
present, the digitalSignature bit must be set for the key to be present, the digitalSignature bit must be set for the key to be
eligible for signing, as described above, and the keyEncipherment eligible for signing, as described above, and the keyEncipherment bit
bit must be present to allow encryption, as described above. The must be present to allow encryption, as described above. The
keyAgreement bit must be set on Diffie-Hellman certificates. keyAgreement bit must be set on Diffie-Hellman certificates.
As CipherSuites which specify new key exchange methods are specified As CipherSuites which specify new key exchange methods are specified
for the TLS Protocol, they will imply certificate format and the for the TLS Protocol, they will imply certificate format and the
required encoded keying information. required encoded keying information.
Structure of this message: Structure of this message:
opaque ASN.1Cert<1..2^24-1>; opaque ASN.1Cert<1..2^24-1>;
struct { struct {
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digitally-signed struct { digitally-signed struct {
opaque sha_hash[20]; opaque sha_hash[20];
}; };
} Signature; } Signature;
7.4.4. Certificate request 7.4.4. Certificate request
When this message will be sent: When this message will be sent:
A non-anonymous server can optionally request a certificate from A non-anonymous server can optionally request a certificate from
the client, if appropriate for the selected cipher suite. This the client, if appropriate for the selected cipher suite. This
message, if sent, will immediately follow the Server Key message, if sent, will immediately follow the Server Key Exchange
Exchange message (if it is sent; otherwise, the Server message (if it is sent; otherwise, the Server Certificate
Certificate message). message).
Structure of this message: Structure of this message:
enum { enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
(255) (255)
} ClientCertificateType; } ClientCertificateType;
opaque DistinguishedName<1..2^16-1>; opaque DistinguishedName<1..2^16-1>;
struct { struct {
ClientCertificateType certificate_types<1..2^8-1>; ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<3..2^16-1>; DistinguishedName certificate_authorities<3..2^16-1>;
} CertificateRequest; } CertificateRequest;
certificate_types certificate_types
This field is a list of the types of certificates requested, This field is a list of the types of certificates requested,
sorted in order of the server's preference. sorted in order of the server's preference.
certificate_authorities certificate_authorities
A list of the distinguished names of acceptable certificate A list of the distinguished names of acceptable certificate
authorities. These distinguished names may specify a desired authorities. These distinguished names may specify a desired
distinguished name for a root CA or for a subordinate CA; distinguished name for a root CA or for a subordinate CA;
thus, this message can be used both to describe known roots thus, this message can be used both to describe known roots
and a desired authorization space. and a desired authorization space.
Note: DistinguishedName is derived from [X509]. Note: DistinguishedName is derived from [X509].
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Diffie-Hellman group and generator encoded in the client's Diffie-Hellman group and generator encoded in the client's
certificate must match the server specified Diffie-Hellman certificate must match the server specified Diffie-Hellman
parameters if the client's parameters are to be used for the key parameters if the client's parameters are to be used for the key
exchange. exchange.
7.4.7. Client key exchange message 7.4.7. Client key exchange message
When this message will be sent: When this message will be sent:
This message is always sent by the client. It will immediately This message is always sent by the client. It will immediately
follow the client certificate message, if it is sent. Otherwise follow the client certificate message, if it is sent. Otherwise
it will be the first message sent by the client after it it will be the first message sent by the client after it receives
receives the server hello done message. the server hello done message.
Meaning of this message: Meaning of this message:
With this message, the premaster secret is set, either though With this message, the premaster secret is set, either though
direct transmission of the RSA-encrypted secret, or by the direct transmission of the RSA-encrypted secret, or by the
transmission of Diffie-Hellman parameters which will allow each transmission of Diffie-Hellman parameters which will allow each
side to agree upon the same premaster secret. When the key side to agree upon the same premaster secret. When the key
exchange method is DH_RSA or DH_DSS, client certification has exchange method is DH_RSA or DH_DSS, client certification has
been requested, and the client was able to respond with a been requested, and the client was able to respond with a
certificate which contained a Diffie-Hellman public key whose certificate which contained a Diffie-Hellman public key whose
parameters (group and generator) matched those specified by the parameters (group and generator) matched those specified by the
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Structure of this message: Structure of this message:
The choice of messages depends on which key exchange method has The choice of messages depends on which key exchange method has
been selected. See Section 7.4.3 for the KeyExchangeAlgorithm been selected. See Section 7.4.3 for the KeyExchangeAlgorithm
definition. definition.
struct { struct {
select (KeyExchangeAlgorithm) { select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret; case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic; case diffie_hellman: ClientDiffieHellmanPublic;
} exchange_keys; } exchange_keys;
} ClientKeyExchange; } ClientKeyExchange;
7.4.7.1. RSA encrypted premaster secret message 7.4.7.1. RSA encrypted premaster secret message
Meaning of this message: Meaning of this message:
If RSA is being used for key agreement and authentication, the If RSA is being used for key agreement and authentication, the
client generates a 48-byte premaster secret, encrypts it using client generates a 48-byte premaster secret, encrypts it using
the public key from the server's certificate or the temporary the public key from the server's certificate or the temporary RSA
RSA key provided in a server key exchange message, and sends the key provided in a server key exchange message, and sends the
result in an encrypted premaster secret message. This structure result in an encrypted premaster secret message. This structure
is a variant of the client key exchange message, not a message is a variant of the client key exchange message, not a message in
in itself. itself.
Structure of this message: Structure of this message:
struct { struct {
ProtocolVersion client_version; ProtocolVersion client_version;
opaque random[46]; opaque random[46];
} PreMasterSecret; } PreMasterSecret;
client_version client_version
The latest (newest) version supported by the client. This is The latest (newest) version supported by the client. This is
used to detect version roll-back attacks. Upon receiving the used to detect version roll-back attacks. Upon receiving the
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} EncryptedPreMasterSecret; } EncryptedPreMasterSecret;
Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used
to attack a TLS server which is using PKCS#1 encoded RSA. The to attack a TLS server which is using PKCS#1 encoded RSA. The
attack takes advantage of the fact that by failing in different attack takes advantage of the fact that by failing in different
ways, a TLS server can be coerced into revealing whether a ways, a TLS server can be coerced into revealing whether a
particular message, when decrypted, is properly PKCS#1 formatted particular message, when decrypted, is properly PKCS#1 formatted
or not. or not.
The best way to avoid vulnerability to this attack is to treat The best way to avoid vulnerability to this attack is to treat
incorrectly formatted messages in a manner indistinguishable incorrectly formatted messages in a manner indistinguishable from
from correctly formatted RSA blocks. Thus, when it receives an correctly formatted RSA blocks. Thus, when it receives an
incorrectly formatted RSA block, a server should generate a incorrectly formatted RSA block, a server should generate a
random 48-byte value and proceed using it as the premaster random 48-byte value and proceed using it as the premaster
secret. Thus, the server will act identically whether the secret. Thus, the server will act identically whether the
received RSA block is correctly encoded or not. received RSA block is correctly encoded or not.
pre_master_secret pre_master_secret
This random value is generated by the client and is used to This random value is generated by the client and is used to
generate the master secret, as specified in Section 8.1. generate the master secret, as specified in Section 8.1.
7.4.7.2. Client Diffie-Hellman public value 7.4.7.2. Client Diffie-Hellman public value
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case explicit: opaque dh_Yc<1..2^16-1>; case explicit: opaque dh_Yc<1..2^16-1>;
} dh_public; } dh_public;
} ClientDiffieHellmanPublic; } ClientDiffieHellmanPublic;
dh_Yc dh_Yc
The client's Diffie-Hellman public value (Yc). The client's Diffie-Hellman public value (Yc).
7.4.8. Certificate verify 7.4.8. Certificate verify
When this message will be sent: When this message will be sent:
This message is used to provide explicit verification of a This message is used to provide explicit verification of a client
client certificate. This message is only sent following a client certificate. This message is only sent following a client
certificate that has signing capability (i.e. all certificates certificate that has signing capability (i.e. all certificates
except those containing fixed Diffie-Hellman parameters). When except those containing fixed Diffie-Hellman parameters). When
sent, it will immediately follow the client key exchange sent, it will immediately follow the client key exchange message.
message.
Structure of this message: Structure of this message:
struct { struct {
Signature signature; Signature signature;
} CertificateVerify; } CertificateVerify;
The Signature type is defined in 7.4.3.
The Signature type is defined in 6.4.3.
CertificateVerify.signature.md5_hash CertificateVerify.signature.md5_hash
MD5(handshake_messages); MD5(handshake_messages);
Certificate.signature.sha_hash Certificate.signature.sha_hash
SHA(handshake_messages); SHA(handshake_messages);
Here handshake_messages refers to all handshake messages sent or Here handshake_messages refers to all handshake messages sent or
received starting at client hello up to but not including this received starting at client hello up to but not including this
message, including the type and length fields of the handshake message, including the type and length fields of the handshake
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7.4.9. Finished 7.4.9. Finished
When this message will be sent: When this message will be sent:
A finished message is always sent immediately after a change A finished message is always sent immediately after a change
cipher spec message to verify that the key exchange and cipher spec message to verify that the key exchange and
authentication processes were successful. It is essential that a authentication processes were successful. It is essential that a
change cipher spec message be received between the other change cipher spec message be received between the other
handshake messages and the Finished message. handshake messages and the Finished message.
Meaning of this message: Meaning of this message:
The finished message is the first protected with the The finished message is the first protected with the just-
just-negotiated algorithms, keys, and secrets. Recipients of negotiated algorithms, keys, and secrets. Recipients of finished
finished messages must verify that the contents are correct. messages must verify that the contents are correct. Once a side
Once a side has sent its Finished message and received and has sent its Finished message and received and validated the
validated the Finished message from its peer, it may begin to Finished message from its peer, it may begin to send and receive
send and receive application data over the connection. application data over the connection.
struct { struct {
opaque verify_data[12]; opaque verify_data[12];
} Finished; } Finished;
verify_data verify_data
PRF(master_secret, finished_label, MD5(handshake_messages) + PRF(master_secret, finished_label, MD5(handshake_messages) +
SHA-1(handshake_messages)) [0..11]; SHA-1(handshake_messages)) [0..11];
finished_label finished_label
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finished_label finished_label
For Finished messages sent by the client, the string "client For Finished messages sent by the client, the string "client
finished". For Finished messages sent by the server, the finished". For Finished messages sent by the server, the
string "server finished". string "server finished".
handshake_messages handshake_messages
All of the data from all handshake messages up to but not All of the data from all handshake messages up to but not
including this message. This is only data visible at the including this message. This is only data visible at the
handshake layer and does not include record layer headers. handshake layer and does not include record layer headers.
This is the concatenation of all the Handshake structures as This is the concatenation of all the Handshake structures as
defined in 7.4 exchanged thus far. defined in 7.4 exchanged thus far.
It is a fatal error if a finished message is not preceded by a It is a fatal error if a finished message is not preceded by a change
change cipher spec message at the appropriate point in the cipher spec message at the appropriate point in the handshake.
handshake.
The hash contained in finished messages sent by the server The hash contained in finished messages sent by the server
incorporate Sender.server; those sent by the client incorporate incorporate Sender.server; those sent by the client incorporate
Sender.client. The value handshake_messages includes all handshake Sender.client. The value handshake_messages includes all handshake
messages starting at client hello up to, but not including, this messages starting at client hello up to, but not including, this
finished message. This may be different from handshake_messages in finished message. This may be different from handshake_messages in
Section 7.4.8 because it would include the certificate verify Section 7.4.8 because it would include the certificate verify message
message (if sent). Also, the handshake_messages for the finished (if sent). Also, the handshake_messages for the finished message sent
message sent by the client will be different from that for the by the client will be different from that for the finished message
finished message sent by the server, because the one which is sent sent by the server, because the one which is sent second will include
second will include the prior one. the prior one.
Note: Change cipher spec messages, alerts and any other record types Note: Change cipher spec messages, alerts and any other record types
are not handshake messages and are not included in the hash are not handshake messages and are not included in the hash
computations. Also, Hello Request messages are omitted from computations. Also, Hello Request messages are omitted from
handshake hashes. handshake hashes.
8. Cryptographic computations 8. Cryptographic computations
In order to begin connection protection, the TLS Record Protocol In order to begin connection protection, the TLS Record Protocol
requires specification of a suite of algorithms, a master secret, requires specification of a suite of algorithms, a master secret, and
and the client and server random values. The authentication, the client and server random values. The authentication, encryption,
encryption, and MAC algorithms are determined by the cipher_suite and MAC algorithms are determined by the cipher_suite selected by the
selected by the server and revealed in the server hello message. The server and revealed in the server hello message. The compression
compression algorithm is negotiated in the hello messages, and the algorithm is negotiated in the hello messages, and the random values
random values are exchanged in the hello messages. All that remains are exchanged in the hello messages. All that remains is to calculate
is to calculate the master secret. the master secret.
8.1. Computing the master secret 8.1. Computing the master secret
For all key exchange methods, the same algorithm is used to convert For all key exchange methods, the same algorithm is used to convert
the pre_master_secret into the master_secret. The pre_master_secret the pre_master_secret into the master_secret. The pre_master_secret
should be deleted from memory once the master_secret has been should be deleted from memory once the master_secret has been
computed. computed.
master_secret = PRF(pre_master_secret, "master secret", master_secret = PRF(pre_master_secret, "master secret",
ClientHello.random + ServerHello.random) ClientHello.random + ServerHello.random)
[0..47]; [0..47];
The master secret is always exactly 48 bytes in length. The length The master secret is always exactly 48 bytes in length. The length of
of the premaster secret will vary depending on key exchange method. the premaster secret will vary depending on key exchange method.
8.1.1. RSA 8.1.1. RSA
When RSA is used for server authentication and key exchange, a When RSA is used for server authentication and key exchange, a 48-
48-byte pre_master_secret is generated by the client, encrypted byte pre_master_secret is generated by the client, encrypted under
under the server's public key, and sent to the server. The server the server's public key, and sent to the server. The server uses its
uses its private key to decrypt the pre_master_secret. Both parties private key to decrypt the pre_master_secret. Both parties then
then convert the pre_master_secret into the master_secret, as convert the pre_master_secret into the master_secret, as specified
specified above. above.
RSA digital signatures are performed using PKCS #1 [PKCS1] block RSA digital signatures are performed using PKCS #1 [PKCS1] block type
type 1. RSA public key encryption is performed using PKCS #1 block 1. RSA public key encryption is performed using PKCS #1 block type 2.
type 2.
8.1.2. Diffie-Hellman 8.1.2. Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The A conventional Diffie-Hellman computation is performed. The
negotiated key (Z) is used as the pre_master_secret, and is negotiated key (Z) is used as the pre_master_secret, and is converted
converted into the master_secret, as specified above. into the master_secret, as specified above.
Note: Diffie-Hellman parameters are specified by the server, and may Note: Diffie-Hellman parameters are specified by the server, and may
be either ephemeral or contained within the server's be either ephemeral or contained within the server's certificate.
certificate.
9. Mandatory Cipher Suites 9. Mandatory Cipher Suites
In the absence of an application profile standard specifying In the absence of an application profile standard specifying
otherwise, a TLS compliant application MUST implement the cipher otherwise, a TLS compliant application MUST implement the cipher
suite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA. suite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA.
10. Application data protocol 10. Application data protocol
Application data messages are carried by the Record Layer and are Application data messages are carried by the Record Layer and are
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struct { struct {
opaque verify_data[12]; opaque verify_data[12];
} Finished; } Finished;
A.5. The CipherSuite A.5. The CipherSuite
The following values define the CipherSuite codes used in the client The following values define the CipherSuite codes used in the client
hello and server hello messages. hello and server hello messages.
A CipherSuite defines a cipher specification supported in TLS A CipherSuite defines a cipher specification supported in TLS Version
Version 1.0. 1.0.
TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
TLS connection during the first handshake on that channel, but must TLS connection during the first handshake on that channel, but must
not be negotiated, as it provides no more protection than an not be negotiated, as it provides no more protection than an
unsecured connection. unsecured connection.
CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 }; CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 };
The following CipherSuite definitions require that the server The following CipherSuite definitions require that the server provide
provide an RSA certificate that can be used for key exchange. The an RSA certificate that can be used for key exchange. The server may
server may request either an RSA or a DSS signature-capable request either an RSA or a DSS signature-capable certificate in the
certificate in the certificate request message. certificate request message.
CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 }; CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 }; CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 };
CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 }; CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 };
CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 }; CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 }; CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 }; CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 };
CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 }; CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 }; CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 };
CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 }; CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A }; CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A };
The following CipherSuite definitions are used for The following CipherSuite definitions are used for server-
server-authenticated (and optionally client-authenticated) authenticated (and optionally client-authenticated) Diffie-Hellman.
Diffie-Hellman. DH denotes cipher suites in which the server's DH denotes cipher suites in which the server's certificate contains
certificate contains the Diffie-Hellman parameters signed by the the Diffie-Hellman parameters signed by the certificate authority
certificate authority (CA). DHE denotes ephemeral Diffie-Hellman, (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
where the Diffie-Hellman parameters are signed by a DSS or RSA parameters are signed by a DSS or RSA certificate, which has been
certificate, which has been signed by the CA. The signing algorithm signed by the CA. The signing algorithm used is specified after the
used is specified after the DH or DHE parameter. The server can DH or DHE parameter. The server can request an RSA or DSS signature-
request an RSA or DSS signature-capable certificate from the client capable certificate from the client for client authentication or it
for client authentication or it may request a Diffie-Hellman may request a Diffie-Hellman certificate. Any Diffie-Hellman
certificate. Any Diffie-Hellman certificate provided by the client certificate provided by the client must use the parameters (group and
must use the parameters (group and generator) described by the generator) described by the server.
server.
CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B }; CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B };
CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C }; CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C };
CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D }; CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D };
CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E }; CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E };
CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F }; CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F };
CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 }; CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 };
CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 }; CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 };
CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 }; CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 };
CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 }; CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 };
CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 }; CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 };
CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 }; CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 };
CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 }; CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 };
The following cipher suites are used for completely anonymous The following cipher suites are used for completely anonymous
Diffie-Hellman communications in which neither party is Diffie-Hellman communications in which neither party is
authenticated. Note that this mode is vulnerable to authenticated. Note that this mode is vulnerable to man-in-the-middle
man-in-the-middle attacks and is therefore deprecated. attacks and is therefore deprecated.
CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 }; CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 };
CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 }; CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 }; CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 };
CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A }; CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B }; CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
Note: All cipher suites whose first byte is 0xFF are considered Note: All cipher suites whose first byte is 0xFF are considered
private and can be used for defining local/experimental private and can be used for defining local/experimental
algorithms. Interoperability of such types is a local matter. algorithms. Interoperability of such types is a local matter.
Note: Additional cipher suites can be registered by publishing an RFC Note: Additional cipher suites can be registered by publishing an RFC
which specifies the cipher suites, including the necessary TLS which specifies the cipher suites, including the necessary TLS
protocol information, including message encoding, premaster protocol information, including message encoding, premaster
secret derivation, symmetric encryption and MAC calculation and secret derivation, symmetric encryption and MAC calculation and
appropriate reference information for the algorithms involved. appropriate reference information for the algorithms involved.
The RFC editor's office may, at its discretion, choose to The RFC editor's office may, at its discretion, choose to publish
publish specifications for cipher suites which are not specifications for cipher suites which are not completely
completely described (e.g., for classified algorithms) if it described (e.g., for classified algorithms) if it finds the
finds the specification to be of technical interest and specification to be of technical interest and completely
completely specified. specified.
Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in reserved to avoid collision with Fortezza-based cipher suites in
SSL 3. SSL 3.
A.6. The Security Parameters A.6. The Security Parameters
These security parameters are determined by the TLS Handshake These security parameters are determined by the TLS Handshake
Protocol and provided as parameters to the TLS Record Layer in order Protocol and provided as parameters to the TLS Record Layer in order
to initialize a connection state. SecurityParameters includes: to initialize a connection state. SecurityParameters includes:
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authentication authentication
Authentication is the ability of one entity to determine the Authentication is the ability of one entity to determine the
identity of another entity. identity of another entity.
block cipher block cipher
A block cipher is an algorithm that operates on plaintext in A block cipher is an algorithm that operates on plaintext in
groups of bits, called blocks. 64 bits is a common block size. groups of bits, called blocks. 64 bits is a common block size.
bulk cipher bulk cipher
A symmetric encryption algorithm used to encrypt large A symmetric encryption algorithm used to encrypt large quantities
quantities of data. of data.
cipher block chaining (CBC) cipher block chaining (CBC)
CBC is a mode in which every plaintext block encrypted with a CBC is a mode in which every plaintext block encrypted with a
block cipher is first exclusive-ORed with the previous block cipher is first exclusive-ORed with the previous ciphertext
ciphertext block (or, in the case of the first block, with the block (or, in the case of the first block, with the
initialization vector). For decryption, every block is first initialization vector). For decryption, every block is first
decrypted, then exclusive-ORed with the previous ciphertext decrypted, then exclusive-ORed with the previous ciphertext block
block (or IV). (or IV).
certificate certificate
As part of the X.509 protocol (a.k.a. ISO Authentication As part of the X.509 protocol (a.k.a. ISO Authentication
framework), certificates are assigned by a trusted Certificate framework), certificates are assigned by a trusted Certificate
Authority and provide a strong binding between a party's Authority and provide a strong binding between a party's identity
identity or some other attributes and its public key. or some other attributes and its public key.
client client
The application entity that initiates a TLS connection to a The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational underlying transport connection. The primary operational
difference between the server and client is that the server is difference between the server and client is that the server is
generally authenticated, while the client is only optionally generally authenticated, while the client is only optionally
authenticated. authenticated.
client write key client write key
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connection connection
A connection is a transport (in the OSI layering model A connection is a transport (in the OSI layering model
definition) that provides a suitable type of service. For TLS, definition) that provides a suitable type of service. For TLS,
such connections are peer to peer relationships. The connections such connections are peer to peer relationships. The connections
are transient. Every connection is associated with one session. are transient. Every connection is associated with one session.
Data Encryption Standard Data Encryption Standard
DES is a very widely used symmetric encryption algorithm. DES is DES is a very widely used symmetric encryption algorithm. DES is
a block cipher with a 56 bit key and an 8 byte block size. Note a block cipher with a 56 bit key and an 8 byte block size. Note
that in TLS, for key generation purposes, DES is treated as that in TLS, for key generation purposes, DES is treated as
having an 8 byte key length (64 bits), but it still only having an 8 byte key length (64 bits), but it still only provides
provides 56 bits of protection. (The low bit of each key byte is 56 bits of protection. (The low bit of each key byte is presumed
presumed to be set to produce odd parity in that key byte.) DES to be set to produce odd parity in that key byte.) DES can also
can also be operated in a mode where three independent keys and be operated in a mode where three independent keys and three
three encryptions are used for each block of data; this uses 168 encryptions are used for each block of data; this uses 168 bits
bits of key (24 bytes in the TLS key generation method) and of key (24 bytes in the TLS key generation method) and provides
provides the equivalent of 112 bits of security. [DES], [3DES] the equivalent of 112 bits of security. [DES], [3DES]
Digital Signature Standard (DSS) Digital Signature Standard (DSS)
A standard for digital signing, including the Digital Signing A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186, "Digital Signature Technology, defined in NIST FIPS PUB 186, "Digital Signature
Standard," published May, 1994 by the U.S. Dept. of Commerce. Standard," published May, 1994 by the U.S. Dept. of Commerce.
[DSS] [DSS]
digital signatures digital signatures
Digital signatures utilize public key cryptography and one-way Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate. authenticated, and is difficult to forge or repudiate.
handshake handshake
An initial negotiation between client and server that An initial negotiation between client and server that establishes
establishes the parameters of their transactions. the parameters of their transactions.
Initialization Vector (IV) Initialization Vector (IV)
When a block cipher is used in CBC mode, the initialization When a block cipher is used in CBC mode, the initialization
vector is exclusive-ORed with the first plaintext block prior to vector is exclusive-ORed with the first plaintext block prior to
encryption. encryption.
IDEA IDEA
A 64-bit block cipher designed by Xuejia Lai and James Massey. A 64-bit block cipher designed by Xuejia Lai and James Massey.
[IDEA] [IDEA]
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master secret master secret
Secure secret data used for generating encryption keys, MAC Secure secret data used for generating encryption keys, MAC
secrets, and IVs. secrets, and IVs.
MD5 MD5
MD5 is a secure hashing function that converts an arbitrarily MD5 is a secure hashing function that converts an arbitrarily
long data stream into a digest of fixed size (16 bytes). [MD5] long data stream into a digest of fixed size (16 bytes). [MD5]
public key cryptography public key cryptography
A class of cryptographic techniques employing two-key ciphers. A class of cryptographic techniques employing two-key ciphers.
Messages encrypted with the public key can only be decrypted Messages encrypted with the public key can only be decrypted with
with the associated private key. Conversely, messages signed the associated private key. Conversely, messages signed with the
with the private key can be verified with the public key. private key can be verified with the public key.
one-way hash function one-way hash function
A one-way transformation that converts an arbitrary amount of A one-way transformation that converts an arbitrary amount of
data into a fixed-length hash. It is computationally hard to data into a fixed-length hash. It is computationally hard to
reverse the transformation or to find collisions. MD5 and SHA reverse the transformation or to find collisions. MD5 and SHA are
are examples of one-way hash functions. examples of one-way hash functions.
RC2 RC2
A block cipher developed by Ron Rivest at RSA Data Security, A block cipher developed by Ron Rivest at RSA Data Security, Inc.
Inc. [RSADSI] described in [RC2]. [RSADSI] described in [RC2].
RC4 RC4
A stream cipher licensed by RSA Data Security [RSADSI]. A A stream cipher licensed by RSA Data Security [RSADSI]. A
compatible cipher is described in [RC4]. compatible cipher is described in [RC4].
RSA RSA
A very widely used public-key algorithm that can be used for A very widely used public-key algorithm that can be used for
either encryption or digital signing. [RSA] either encryption or digital signing. [RSA]
salt salt
Non-secret random data used to make export encryption keys Non-secret random data used to make export encryption keys resist
resist precomputation attacks. precomputation attacks.
server server
The server is the application entity that responds to requests The server is the application entity that responds to requests
for connections from clients. See also under client. for connections from clients. See also under client.
session session
A TLS session is an association between a client and a server. A TLS session is an association between a client and a server.
Sessions are created by the handshake protocol. Sessions define Sessions are created by the handshake protocol. Sessions define a
a set of cryptographic security parameters, which can be shared set of cryptographic security parameters, which can be shared
among multiple connections. Sessions are used to avoid the among multiple connections. Sessions are used to avoid the
expensive negotiation of new security parameters for each expensive negotiation of new security parameters for each
connection. connection.
session identifier session identifier
A session identifier is a value generated by a server that A session identifier is a value generated by a server that
identifies a particular session. identifies a particular session.
server write key server write key
The key used to encrypt data written by the server. The key used to encrypt data written by the server.
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The amount of data a block cipher enciphers in one chunk; a The amount of data a block cipher enciphers in one chunk; a
block cipher running in CBC mode can only encrypt an even block cipher running in CBC mode can only encrypt an even
multiple of its block size. multiple of its block size.
Hash Hash Padding Hash Hash Padding
function Size Size function Size Size
NULL 0 0 NULL 0 0
MD5 16 48 MD5 16 48
SHA 20 40 SHA 20 40
Appendix D
D. Implementation Notes D. Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors. section provides several recommendations to assist implementors.
D.1. Temporary RSA keys D.1. Temporary RSA keys
US Export restrictions limit RSA keys used for encryption to 512 US Export restrictions limit RSA keys used for encryption to 512
bits, but do not place any limit on lengths of RSA keys used for bits, but do not place any limit on lengths of RSA keys used for
signing operations. Certificates often need to be larger than 512 signing operations. Certificates often need to be larger than 512
bits, since 512-bit RSA keys are not secure enough for high-value bits, since 512-bit RSA keys are not secure enough for high-value
transactions or for applications requiring long-term security. Some transactions or for applications requiring long-term security. Some
certificates are also designated signing-only, in which case they certificates are also designated signing-only, in which case they
cannot be used for key exchange. cannot be used for key exchange.
When the public key in the certificate cannot be used for When the public key in the certificate cannot be used for encryption,
encryption, the server signs a temporary RSA key, which is then the server signs a temporary RSA key, which is then exchanged. In
exchanged. In exportable applications, the temporary RSA key should exportable applications, the temporary RSA key should be the maximum
be the maximum allowable length (i.e., 512 bits). Because 512-bit allowable length (i.e., 512 bits). Because 512-bit RSA keys are
RSA keys are relatively insecure, they should be changed often. For relatively insecure, they should be changed often. For typical
typical electronic commerce applications, it is suggested that keys electronic commerce applications, it is suggested that keys be
be changed daily or every 500 transactions, and more often if changed daily or every 500 transactions, and more often if possible.
possible. Note that while it is acceptable to use the same temporary Note that while it is acceptable to use the same temporary key for
key for multiple transactions, it must be signed each time it is multiple transactions, it must be signed each time it is used.
used.
RSA key generation is a time-consuming process. In many cases, a RSA key generation is a time-consuming process. In many cases, a
low-priority process can be assigned the task of key generation. low-priority process can be assigned the task of key generation.
Whenever a new key is completed, the existing temporary key can be Whenever a new key is completed, the existing temporary key can be
replaced with the new one. replaced with the new one.
D.2. Random Number Generation and Seeding D.2. Random Number Generation and Seeding
TLS requires a cryptographically-secure pseudorandom number TLS requires a cryptographically-secure pseudorandom number generator
generator (PRNG). Care must be taken in designing and seeding PRNGs. (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs
PRNGs based on secure hash operations, most notably MD5 and/or SHA, based on secure hash operations, most notably MD5 and/or SHA, are
are acceptable, but cannot provide more security than the size of acceptable, but cannot provide more security than the size of the
the random number generator state. (For example, MD5-based PRNGs random number generator state. (For example, MD5-based PRNGs usually
usually provide 128 bits of state.) provide 128 bits of state.)
To estimate the amount of seed material being produced, add the To estimate the amount of seed material being produced, add the
number of bits of unpredictable information in each seed byte. For number of bits of unpredictable information in each seed byte. For
example, keystroke timing values taken from a PC compatible's 18.2 example, keystroke timing values taken from a PC compatible's 18.2 Hz
Hz timer provide 1 or 2 secure bits each, even though the total size timer provide 1 or 2 secure bits each, even though the total size of
of the counter value is 16 bits or more. To seed a 128-bit PRNG, one the counter value is 16 bits or more. To seed a 128-bit PRNG, one
would thus require approximately 100 such timer values. would thus require approximately 100 such timer values.
Warning: The seeding functions in RSAREF and versions of BSAFE prior to Warning: The seeding functions in RSAREF and versions of BSAFE prior to
3.0 are order-independent. For example, if 1000 seed bits are 3.0 are order-independent. For example, if 1000 seed bits are
supplied, one at a time, in 1000 separate calls to the seed supplied, one at a time, in 1000 separate calls to the seed
function, the PRNG will end up in a state which depends only function, the PRNG will end up in a state which depends only
on the number of 0 or 1 seed bits in the seed data (i.e., on the number of 0 or 1 seed bits in the seed data (i.e.,
there are 1001 possible final states). Applications using there are 1001 possible final states). Applications using
BSAFE or RSAREF must take extra care to ensure proper seeding. BSAFE or RSAREF must take extra care to ensure proper seeding.
This may be accomplished by accumulating seed bits into a This may be accomplished by accumulating seed bits into a
buffer and processing them all at once or by processing an buffer and processing them all at once or by processing an
incrementing counter with every seed bit; either method will incrementing counter with every seed bit; either method will
reintroduce order dependence into the seeding process. reintroduce order dependence into the seeding process.
D.3. Certificates and authentication D.3. Certificates and authentication
Implementations are responsible for verifying the integrity of Implementations are responsible for verifying the integrity of
certificates and should generally support certificate revocation certificates and should generally support certificate revocation
messages. Certificates should always be verified to ensure proper messages. Certificates should always be verified to ensure proper
signing by a trusted Certificate Authority (CA). The selection and signing by a trusted Certificate Authority (CA). The selection and
addition of trusted CAs should be done very carefully. Users should addition of trusted CAs should be done very carefully. Users should
be able to view information about the certificate and root CA. be able to view information about the certificate and root CA.
D.4. CipherSuites D.4. CipherSuites
TLS supports a range of key sizes and security levels, including TLS supports a range of key sizes and security levels, including some
some which provide no or minimal security. A proper implementation which provide no or minimal security. A proper implementation will
will probably not support many cipher suites. For example, 40-bit probably not support many cipher suites. For example, 40-bit
encryption is easily broken, so implementations requiring strong encryption is easily broken, so implementations requiring strong
security should not allow 40-bit keys. Similarly, anonymous security should not allow 40-bit keys. Similarly, anonymous Diffie-
Diffie-Hellman is strongly discouraged because it cannot prevent Hellman is strongly discouraged because it cannot prevent man-in-
man-in-the-middle attacks. Applications should also enforce minimum the-middle attacks. Applications should also enforce minimum and
and maximum key sizes. For example, certificate chains containing maximum key sizes. For example, certificate chains containing 512-bit
512-bit RSA keys or signatures are not appropriate for high-security RSA keys or signatures are not appropriate for high-security
applications. applications.
E. Backward Compatibility With SSL E. Backward Compatibility With SSL
For historical reasons and in order to avoid a profligate For historical reasons and in order to avoid a profligate consumption
consumption of reserved port numbers, application protocols which of reserved port numbers, application protocols which are secured by
are secured by TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same
the same connection port: for example, the https protocol (HTTP connection port: for example, the https protocol (HTTP secured by SSL
secured by SSL or TLS) uses port 443 regardless of which security or TLS) uses port 443 regardless of which security protocol it is
protocol it is using. Thus, some mechanism must be determined to using. Thus, some mechanism must be determined to distinguish and
distinguish and negotiate among the various protocols. negotiate among the various protocols.
TLS version 1.0 and SSL 3.0 are very similar; thus, supporting both TLS version 1.0 and SSL 3.0 are very similar; thus, supporting both
is easy. TLS clients who wish to negotiate with SSL 3.0 servers is easy. TLS clients who wish to negotiate with SSL 3.0 servers
should send client hello messages using the SSL 3.0 record format should send client hello messages using the SSL 3.0 record format and
and client hello structure, sending {3, 1} for the version field to client hello structure, sending {3, 1} for the version field to note
note that they support TLS 1.0. If the server supports only SSL 3.0, that they support TLS 1.0. If the server supports only SSL 3.0, it
it will respond with an SSL 3.0 server hello; if it supports TLS, will respond with an SSL 3.0 server hello; if it supports TLS, with a
with a TLS server hello. The negotiation then proceeds as TLS server hello. The negotiation then proceeds as appropriate for
appropriate for the negotiated protocol. the negotiated protocol.
Similarly, a TLS server which wishes to interoperate with SSL 3.0 Similarly, a TLS server which wishes to interoperate with SSL 3.0
clients should accept SSL 3.0 client hello messages and respond with clients should accept SSL 3.0 client hello messages and respond with
an SSL 3.0 server hello if an SSL 3.0 client hello is received which an SSL 3.0 server hello if an SSL 3.0 client hello is received which
has a version field of {3, 0}, denoting that this client does not has a version field of {3, 0}, denoting that this client does not
support TLS. support TLS.
Whenever a client already knows the highest protocol known to a Whenever a client already knows the highest protocol known to a
server (for example, when resuming a session), it should initiate server (for example, when resuming a session), it should initiate the
the connection in that native protocol. connection in that native protocol.
TLS 1.0 clients that support SSL Version 2.0 servers must send SSL TLS 1.0 clients that support SSL Version 2.0 servers must send SSL
Version 2.0 client hello messages [SSL2]. TLS servers should accept Version 2.0 client hello messages [SSL2]. TLS servers should accept
either client hello format if they wish to support SSL 2.0 clients either client hello format if they wish to support SSL 2.0 clients on
on the same connection port. The only deviations from the Version the same connection port. The only deviations from the Version 2.0
2.0 specification are the ability to specify a version with a value specification are the ability to specify a version with a value of
of three and the support for more ciphering types in the CipherSpec. three and the support for more ciphering types in the CipherSpec.
Warning: The ability to send Version 2.0 client hello messages will be Warning: The ability to send Version 2.0 client hello messages will be
phased out with all due haste. Implementors should make every phased out with all due haste. Implementors should make every
effort to move forward as quickly as possible. Version 3.0 effort to move forward as quickly as possible. Version 3.0
provides better mechanisms for moving to newer versions. provides better mechanisms for moving to newer versions.
The following cipher specifications are carryovers from SSL Version The following cipher specifications are carryovers from SSL Version
2.0. These are assumed to use RSA for key exchange and 2.0. These are assumed to use RSA for key exchange and
authentication. authentication.
V2CipherSpec TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 }; V2CipherSpec TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 };
V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 }; V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 }; V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 };
V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5 V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
= { 0x04,0x00,0x80 }; = { 0x04,0x00,0x80 };
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This is a list of all CipherSpecs the client is willing and able This is a list of all CipherSpecs the client is willing and able
to use. There must be at least one CipherSpec acceptable to the to use. There must be at least one CipherSpec acceptable to the
server. server.
session_id session_id
If this field's length is not zero, it will contain the If this field's length is not zero, it will contain the
identification for a session that the client wishes to resume. identification for a session that the client wishes to resume.
challenge challenge
The client challenge to the server for the server to identify The client challenge to the server for the server to identify
itself is a (nearly) arbitrary length random. The TLS server itself is a (nearly) arbitrary length random. The TLS server will
will right justify the challenge data to become the right justify the challenge data to become the ClientHello.random
ClientHello.random data (padded with leading zeroes, if data (padded with leading zeroes, if necessary), as specified in
necessary), as specified in this protocol specification. If the this protocol specification. If the length of the challenge is
length of the challenge is greater than 32 bytes, only the last greater than 32 bytes, only the last 32 bytes are used. It is
32 bytes are used. It is legitimate (but not necessary) for a V3 legitimate (but not necessary) for a V3 server to reject a V2
server to reject a V2 ClientHello that has fewer than 16 bytes ClientHello that has fewer than 16 bytes of challenge data.
of challenge data.
Note: Requests to resume a TLS session should use a TLS client hello. Note: Requests to resume a TLS session should use a TLS client hello.
E.2. Avoiding man-in-the-middle version rollback E.2. Avoiding man-in-the-middle version rollback
When TLS clients fall back to Version 2.0 compatibility mode, they When TLS clients fall back to Version 2.0 compatibility mode, they
should use special PKCS #1 block formatting. This is done so that should use special PKCS #1 block formatting. This is done so that TLS
TLS servers will reject Version 2.0 sessions with TLS-capable servers will reject Version 2.0 sessions with TLS-capable clients.
clients.
When TLS clients are in Version 2.0 compatibility mode, they set the When TLS clients are in Version 2.0 compatibility mode, they set the
right-hand (least-significant) 8 random bytes of the PKCS padding right-hand (least-significant) 8 random bytes of the PKCS padding
(not including the terminal null of the padding) for the RSA (not including the terminal null of the padding) for the RSA
encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY
to 0x03 (the other padding bytes are random). After decrypting the to 0x03 (the other padding bytes are random). After decrypting the
ENCRYPTED-KEY-DATA field, servers that support TLS should issue an ENCRYPTED-KEY-DATA field, servers that support TLS should issue an
error if these eight padding bytes are 0x03. Version 2.0 servers error if these eight padding bytes are 0x03. Version 2.0 servers
receiving blocks padded in this manner will proceed normally. receiving blocks padded in this manner will proceed normally.
Appendix F
F. Security analysis F. Security analysis
The TLS protocol is designed to establish a secure connection The TLS protocol is designed to establish a secure connection between
between a client and a server communicating over an insecure a client and a server communicating over an insecure channel. This
channel. This document makes several traditional assumptions, document makes several traditional assumptions, including that
including that attackers have substantial computational resources attackers have substantial computational resources and cannot obtain
and cannot obtain secret information from sources outside the secret information from sources outside the protocol. Attackers are
protocol. Attackers are assumed to have the ability to capture, assumed to have the ability to capture, modify, delete, replay, and
modify, delete, replay, and otherwise tamper with messages sent over otherwise tamper with messages sent over the communication channel.
the communication channel. This appendix outlines how TLS has been This appendix outlines how TLS has been designed to resist a variety
designed to resist a variety of attacks. of attacks.
F.1. Handshake protocol F.1. Handshake protocol
The handshake protocol is responsible for selecting a CipherSpec and The handshake protocol is responsible for selecting a CipherSpec and
generating a Master Secret, which together comprise the primary generating a Master Secret, which together comprise the primary
cryptographic parameters associated with a secure session. The cryptographic parameters associated with a secure session. The
handshake protocol can also optionally authenticate parties who have handshake protocol can also optionally authenticate parties who have
certificates signed by a trusted certificate authority. certificates signed by a trusted certificate authority.
F.1.1. Authentication and key exchange F.1.1. Authentication and key exchange
TLS supports three authentication modes: authentication of both TLS supports three authentication modes: authentication of both
parties, server authentication with an unauthenticated client, and parties, server authentication with an unauthenticated client, and
total anonymity. Whenever the server is authenticated, the channel total anonymity. Whenever the server is authenticated, the channel is
is secure against man-in-the-middle attacks, but completely secure against man-in-the-middle attacks, but completely anonymous
anonymous sessions are inherently vulnerable to such attacks. sessions are inherently vulnerable to such attacks. Anonymous
Anonymous servers cannot authenticate clients. If the server is servers cannot authenticate clients. If the server is authenticated,
authenticated, its certificate message must provide a valid its certificate message must provide a valid certificate chain
certificate chain leading to an acceptable certificate authority. leading to an acceptable certificate authority. Similarly,
Similarly, authenticated clients must supply an acceptable authenticated clients must supply an acceptable certificate to the
certificate to the server. Each party is responsible for verifying server. Each party is responsible for verifying that the other's
that the other's certificate is valid and has not expired or been certificate is valid and has not expired or been revoked.
revoked.
The general goal of the key exchange process is to create a The general goal of the key exchange process is to create a
pre_master_secret known to the communicating parties and not to pre_master_secret known to the communicating parties and not to
attackers. The pre_master_secret will be used to generate the attackers. The pre_master_secret will be used to generate the
master_secret (see Section 8.1). The master_secret is required to master_secret (see Section 8.1). The master_secret is required to
generate the certificate verify and finished messages, encryption generate the certificate verify and finished messages, encryption
keys, and MAC secrets (see Sections 7.4.8, 7.4.9 and 6.3). By keys, and MAC secrets (see Sections 7.4.8, 7.4.9 and 6.3). By sending
sending a correct finished message, parties thus prove that they a correct finished message, parties thus prove that they know the
know the correct pre_master_secret. correct pre_master_secret.
F.1.1.1. Anonymous key exchange F.1.1.1. Anonymous key exchange
Completely anonymous sessions can be established using RSA or Completely anonymous sessions can be established using RSA or
Diffie-Hellman for key exchange. With anonymous RSA, the client Diffie-Hellman for key exchange. With anonymous RSA, the client
encrypts a pre_master_secret with the server's uncertified public encrypts a pre_master_secret with the server's uncertified public key
key extracted from the server key exchange message. The result is extracted from the server key exchange message. The result is sent in
sent in a client key exchange message. Since eavesdroppers do not a client key exchange message. Since eavesdroppers do not know the
know the server's private key, it will be infeasible for them to server's private key, it will be infeasible for them to decode the
decode the pre_master_secret. (Note that no anonymous RSA Cipher pre_master_secret. (Note that no anonymous RSA Cipher Suites are
Suites are defined in this document). defined in this document).
With Diffie-Hellman, the server's public parameters are contained in With Diffie-Hellman, the server's public parameters are contained in
the server key exchange message and the client's are sent in the the server key exchange message and the client's are sent in the
client key exchange message. Eavesdroppers who do not know the client key exchange message. Eavesdroppers who do not know the
private values should not be able to find the Diffie-Hellman result private values should not be able to find the Diffie-Hellman result
(i.e. the pre_master_secret). (i.e. the pre_master_secret).
Warning: Completely anonymous connections only provide protection Warning: Completely anonymous connections only provide protection
against passive eavesdropping. Unless an independent against passive eavesdropping. Unless an independent tamper-
tamper-proof channel is used to verify that the finished proof channel is used to verify that the finished messages
messages were not replaced by an attacker, server were not replaced by an attacker, server authentication is
authentication is required in environments where active required in environments where active man-in-the-middle
man-in-the-middle attacks are a concern. attacks are a concern.
F.1.1.2. RSA key exchange and authentication F.1.1.2. RSA key exchange and authentication
With RSA, key exchange and server authentication are combined. The With RSA, key exchange and server authentication are combined. The
public key may be either contained in the server's certificate or public key may be either contained in the server's certificate or may
may be a temporary RSA key sent in a server key exchange message. be a temporary RSA key sent in a server key exchange message. When
When temporary RSA keys are used, they are signed by the server's temporary RSA keys are used, they are signed by the server's RSA or
RSA or DSS certificate. The signature includes the current DSS certificate. The signature includes the current
ClientHello.random, so old signatures and temporary keys cannot be ClientHello.random, so old signatures and temporary keys cannot be
replayed. Servers may use a single temporary RSA key for multiple replayed. Servers may use a single temporary RSA key for multiple
negotiation sessions. negotiation sessions.
Note: The temporary RSA key option is useful if servers need large Note: The temporary RSA key option is useful if servers need large
certificates but must comply with government-imposed size limits certificates but must comply with government-imposed size limits
on keys used for key exchange. on keys used for key exchange.
After verifying the server's certificate, the client encrypts a After verifying the server's certificate, the client encrypts a
pre_master_secret with the server's public key. By successfully pre_master_secret with the server's public key. By successfully
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parameters, its certificate contains the information required to parameters, its certificate contains the information required to
complete the key exchange. Note that in this case the client and complete the key exchange. Note that in this case the client and
server will generate the same Diffie-Hellman result (i.e., server will generate the same Diffie-Hellman result (i.e.,
pre_master_secret) every time they communicate. To prevent the pre_master_secret) every time they communicate. To prevent the
pre_master_secret from staying in memory any longer than necessary, pre_master_secret from staying in memory any longer than necessary,
it should be converted into the master_secret as soon as possible. it should be converted into the master_secret as soon as possible.
Client Diffie-Hellman parameters must be compatible with those Client Diffie-Hellman parameters must be compatible with those
supplied by the server for the key exchange to work. supplied by the server for the key exchange to work.
If the client has a standard DSS or RSA certificate or is If the client has a standard DSS or RSA certificate or is
unauthenticated, it sends a set of temporary parameters to the unauthenticated, it sends a set of temporary parameters to the server
server in the client key exchange message, then optionally uses a in the client key exchange message, then optionally uses a
certificate verify message to authenticate itself. certificate verify message to authenticate itself.
F.1.2. Version rollback attacks F.1.2. Version rollback attacks
Because TLS includes substantial improvements over SSL Version 2.0, Because TLS includes substantial improvements over SSL Version 2.0,
attackers may try to make TLS-capable clients and servers fall back attackers may try to make TLS-capable clients and servers fall back
to Version 2.0. This attack can occur if (and only if) two to Version 2.0. This attack can occur if (and only if) two TLS-
TLS-capable parties use an SSL 2.0 handshake. capable parties use an SSL 2.0 handshake.
Although the solution using non-random PKCS #1 block type 2 message Although the solution using non-random PKCS #1 block type 2 message
padding is inelegant, it provides a reasonably secure way for padding is inelegant, it provides a reasonably secure way for Version
Version 3.0 servers to detect the attack. This solution is not 3.0 servers to detect the attack. This solution is not secure against
secure against attackers who can brute force the key and substitute attackers who can brute force the key and substitute a new
a new ENCRYPTED-KEY-DATA message containing the same key (but with ENCRYPTED-KEY-DATA message containing the same key (but with normal
normal padding) before the application specified wait threshold has padding) before the application specified wait threshold has expired.
expired. Parties concerned about attacks of this scale should not be Parties concerned about attacks of this scale should not be using
using 40-bit encryption keys anyway. Altering the padding of the 40-bit encryption keys anyway. Altering the padding of the least-
least-significant 8 bytes of the PKCS padding does not impact significant 8 bytes of the PKCS padding does not impact security for
security for the size of the signed hashes and RSA key lengths used the size of the signed hashes and RSA key lengths used in the
in the protocol, since this is essentially equivalent to increasing protocol, since this is essentially equivalent to increasing the
the input block size by 8 bytes. input block size by 8 bytes.
F.1.3. Detecting attacks against the handshake protocol F.1.3. Detecting attacks against the handshake protocol
An attacker might try to influence the handshake exchange to make An attacker might try to influence the handshake exchange to make the
the parties select different encryption algorithms than they would parties select different encryption algorithms than they would
normally choose. Because many implementations will support 40-bit normally choose. Because many implementations will support 40-bit
exportable encryption and some may even support null encryption or exportable encryption and some may even support null encryption or
MAC algorithms, this attack is of particular concern. MAC algorithms, this attack is of particular concern.
For this attack, an attacker must actively change one or more For this attack, an attacker must actively change one or more
handshake messages. If this occurs, the client and server will handshake messages. If this occurs, the client and server will
compute different values for the handshake message hashes. As a compute different values for the handshake message hashes. As a
result, the parties will not accept each others' finished messages. result, the parties will not accept each others' finished messages.
Without the master_secret, the attacker cannot repair the finished Without the master_secret, the attacker cannot repair the finished
messages, so the attack will be discovered. messages, so the attack will be discovered.
F.1.4. Resuming sessions F.1.4. Resuming sessions
When a connection is established by resuming a session, new When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the ClientHello.random and ServerHello.random values are hashed with the
session's master_secret. Provided that the master_secret has not session's master_secret. Provided that the master_secret has not been
been compromised and that the secure hash operations used to produce compromised and that the secure hash operations used to produce the
the encryption keys and MAC secrets are secure, the connection encryption keys and MAC secrets are secure, the connection should be
should be secure and effectively independent from previous secure and effectively independent from previous connections.
connections. Attackers cannot use known encryption keys or MAC Attackers cannot use known encryption keys or MAC secrets to
secrets to compromise the master_secret without breaking the secure compromise the master_secret without breaking the secure hash
hash operations (which use both SHA and MD5). operations (which use both SHA and MD5).
Sessions cannot be resumed unless both the client and server agree. Sessions cannot be resumed unless both the client and server agree.
If either party suspects that the session may have been compromised, If either party suspects that the session may have been compromised,
or that certificates may have expired or been revoked, it should or that certificates may have expired or been revoked, it should
force a full handshake. An upper limit of 24 hours is suggested for force a full handshake. An upper limit of 24 hours is suggested for
session ID lifetimes, since an attacker who obtains a master_secret session ID lifetimes, since an attacker who obtains a master_secret
may be able to impersonate the compromised party until the may be able to impersonate the compromised party until the
corresponding session ID is retired. Applications that may be run in corresponding session ID is retired. Applications that may be run in
relatively insecure environments should not write session IDs to relatively insecure environments should not write session IDs to
stable storage. stable storage.
F.1.5. MD5 and SHA F.1.5. MD5 and SHA
TLS uses hash functions very conservatively. Where possible, both TLS uses hash functions very conservatively. Where possible, both MD5
MD5 and SHA are used in tandem to ensure that non-catastrophic flaws and SHA are used in tandem to ensure that non-catastrophic flaws in
in one algorithm will not break the overall protocol. one algorithm will not break the overall protocol.
F.2. Protecting application data F.2. Protecting application data
The master_secret is hashed with the ClientHello.random and The master_secret is hashed with the ClientHello.random and
ServerHello.random to produce unique data encryption keys and MAC ServerHello.random to produce unique data encryption keys and MAC
secrets for each connection. secrets for each connection.
Outgoing data is protected with a MAC before transmission. To Outgoing data is protected with a MAC before transmission. To prevent
prevent message replay or modification attacks, the MAC is computed message replay or modification attacks, the MAC is computed from the
from the MAC secret, the sequence number, the message length, the MAC secret, the sequence number, the message length, the message
message contents, and two fixed character strings. The message type contents, and two fixed character strings. The message type field is
field is necessary to ensure that messages intended for one TLS necessary to ensure that messages intended for one TLS Record Layer
Record Layer client are not redirected to another. The sequence client are not redirected to another. The sequence number ensures
number ensures that attempts to delete or reorder messages will be that attempts to delete or reorder messages will be detected. Since
detected. Since sequence numbers are 64-bits long, they should never sequence numbers are 64-bits long, they should never overflow.
overflow. Messages from one party cannot be inserted into the Messages from one party cannot be inserted into the other's output,
other's output, since they use independent MAC secrets. Similarly, since they use independent MAC secrets. Similarly, the server-write
the server-write and client-write keys are independent so stream and client-write keys are independent so stream cipher keys are used
cipher keys are used only once. only once.
If an attacker does break an encryption key, all messages encrypted If an attacker does break an encryption key, all messages encrypted
with it can be read. Similarly, compromise of a MAC key can make with it can be read. Similarly, compromise of a MAC key can make
message modification attacks possible. Because MACs are also message modification attacks possible. Because MACs are also
encrypted, message-alteration attacks generally require breaking the encrypted, message-alteration attacks generally require breaking the
encryption algorithm as well as the MAC. encryption algorithm as well as the MAC.
Note: MAC secrets may be larger than encryption keys, so messages can Note: MAC secrets may be larger than encryption keys, so messages can
remain tamper resistant even if encryption keys are broken. remain tamper resistant even if encryption keys are broken.
F.3. Final notes F.3. Final notes
For TLS to be able to provide a secure connection, both the client For TLS to be able to provide a secure connection, both the client
and server systems, keys, and applications must be secure. In and server systems, keys, and applications must be secure. In
addition, the implementation must be free of security errors. addition, the implementation must be free of security errors.
The system is only as strong as the weakest key exchange and The system is only as strong as the weakest key exchange and
authentication algorithm supported, and only trustworthy authentication algorithm supported, and only trustworthy
cryptographic functions should be used. Short public keys, 40-bit cryptographic functions should be used. Short public keys, 40-bit
bulk encryption keys, and anonymous servers should be used with bulk encryption keys, and anonymous servers should be used with great
great caution. Implementations and users must be careful when caution. Implementations and users must be careful when deciding
deciding which certificates and certificate authorities are which certificates and certificate authorities are acceptable; a
acceptable; a dishonest certificate authority can do tremendous dishonest certificate authority can do tremendous damage.
damage.
Appendix G
G. Patent Statement G. Patent Statement
Some of the cryptographic algorithms proposed for use in this Some of the cryptographic algorithms proposed for use in this
protocol have patent claims on them. In addition Netscape protocol have patent claims on them. In addition Netscape
Communications Corporation has a patent claim on the Secure Sockets Communications Corporation has a patent claim on the Secure Sockets
Layer (SSL) work that this standard is based on. The Internet Layer (SSL) work that this standard is based on. The Internet
Standards Process as defined in RFC 1310 requires a written Standards Process as defined in RFC 2026 requests that a statement be
statement from the Patent holder that a license will be made obtained from a Patent holder indicating that a license will be made
available to applicants under reasonable terms and conditions prior available to applicants under reasonable terms and conditions.
to approving a specification as a Proposed, Draft or Internet
Standard.
The Massachusetts Institute of Technology has granted RSA Data The Massachusetts Institute of Technology has granted RSA Data
Security, Inc., exclusive sub-licensing rights to the following Security, Inc., exclusive sub-licensing rights to the following
patent issued in the United States: patent issued in the United States:
Cryptographic Communications System and Method ("RSA"), No. Cryptographic Communications System and Method ("RSA"), No.
4,405,829 4,405,829
Netscape Communications Corporation has been issued the following Netscape Communications Corporation has been issued the following
patent in the United States: patent in the United States:
skipping to change at page 68, line 11 skipping to change at page 74, line 48
transport protocol with security features. Netscape encourages transport protocol with security features. Netscape encourages
the royalty-free adoption and use of the SSL protocol upon the the royalty-free adoption and use of the SSL protocol upon the
following terms and conditions: following terms and conditions:
* If you already have a valid SSL Ref license today which * If you already have a valid SSL Ref license today which
includes source code from Netscape, an additional patent includes source code from Netscape, an additional patent
license under the SSL patent is not required. license under the SSL patent is not required.
* If you don't have an SSL Ref license, you may have a royalty * If you don't have an SSL Ref license, you may have a royalty
free license to build implementations covered by the SSL free license to build implementations covered by the SSL
Patent Claims or the IETF TLS specification provided that Patent Claims or the IETF TLS specification provided that you
you do not to assert any patent rights against Netscape or do not to assert any patent rights against Netscape or other
other companies for the implementation of SSL or the IETF companies for the implementation of SSL or the IETF TLS
TLS recommendation. recommendation.
What are "Patent Claims": What are "Patent Claims":
Patent claims are claims in an issued foreign or domestic patent Patent claims are claims in an issued foreign or domestic patent
that: that:
1) must be infringed in order to implement methods or build 1) must be infringed in order to implement methods or build
products according to the IETF TLS specification; or products according to the IETF TLS specification; or
2) patent claims which require the elements of the SSL patent 2) patent claims which require the elements of the SSL patent
claims and/or their equivalents to be infringed. claims and/or their equivalents to be infringed.
The Internet Society, Internet Architecture Board, Internet The Internet Society, Internet Architecture Board, Internet
Engineering Steering Group and the Corporation for National Research Engineering Steering Group and the Corporation for National Research
Initiatives take no position on the validity or scope of the patents Initiatives take no position on the validity or scope of the patents
and patent applications, nor on the appropriateness of the terms of and patent applications, nor on the appropriateness of the terms of
the assurance. The Internet Society and other groups mentioned above the assurance. The Internet Society and other groups mentioned above
have not made any determination as to any other intellectual have not made any determination as to any other intellectual property
property rights which may apply to the practice of this standard. rights which may apply to the practice of this standard. Any further
Any further consideration of these matters is the user's own consideration of these matters is the user's own responsibility.
responsibility.
Security Considerations
Security issues are discussed throughout this memo.
References References
[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES," [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41. IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
[BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against [BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against
Protocols Based on RSA Encryption Standard PKCS #1" in Advances in Protocols Based on RSA Encryption Standard PKCS #1" in
Cryptology -- CRYPTO'98, LNCS vol. 1462, pages: 1--12, 1998. Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages:
1--12, 1998.
[DES] ANSI X3.106, "American National Standard for Information [DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption," American National Standards Systems-Data Link Encryption," American National Standards
Institute, 1983. Institute, 1983.
[DH1] W. Diffie and M. E. Hellman, "New Directions in Cryptography," [DH1] W. Diffie and M. E. Hellman, "New Directions in
IEEE Transactions on Information Theory, V. IT-22, n. 6, Jun 1977, Cryptography," IEEE Transactions on Information Theory, V.
pp. 74-84. IT-22, n. 6, Jun 1977, pp. 74-84.
[DSS] NIST FIPS PUB 186, "Digital Signature Standard," National [DSS] NIST FIPS PUB 186, "Digital Signature Standard," National
Institute of Standards and Technology, U.S. Department of Commerce, Institute of Standards and Technology, U.S. Department of
May 18, 1994. Commerce, May 18, 1994.
[FTP] J. Postel and J. Reynolds, RFC 959: File Transfer Protocol, [FTP] Postel J., and J. Reynolds, "File Transfer Protocol", STD 9,
October 1985. RFC 959, October 1985.
[HTTP] T. Berners-Lee, R. Fielding, H. Frystyk, Hypertext Transfer [HTTP] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
Protocol -- HTTP/1.0, October, 1995. Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[HMAC] H. Krawczyk, M. Bellare, and R. Canetti, RFC 2104, HMAC: [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Keyed-Hashing for Message Authentication, February, 1997. Hashing for Message Authentication," RFC 2104, February
1997.
[IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH
Series in Information Processing, v. 1, Konstanz: Hartung-Gorre Series in Information Processing, v. 1, Konstanz: Hartung-
Verlag, 1992. Gorre Verlag, 1992.
[MD2] R. Rivest. RFC 1319: The MD2 Message Digest Algorithm. April [MD2] Kaliski, B., "The MD2 Message Digest Algorithm", RFC 1319,
1992. April 1992.
[MD5] R. Rivest. RFC 1321: The MD5 Message Digest Algorithm. April [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
1992. April 1992.
[PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard," [PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard,"
version 1.5, November 1993. version 1.5, November 1993.
[PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax
Standard," version 1.5, November 1993. Standard," version 1.5, November 1993.
[PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax
Standard," version 1.5, November 1993. Standard," version 1.5, November 1993.
[PKIX] R. Housley, W. Ford, W. Polk, D. Solo, Internet Public Key [PKIX] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
Infrastructure: Part I: X.509 Certificate and CRL Profile, Public Key Infrastructure: Part I: X.509 Certificate and CRL
<draft-ietf-pkix-ipki-part1-06.txt>, October 1997. Profile", RFC 2459, January 1999.
[RC2] R. Rivest, A Description of the RC2(r) Encryption Algorithm [RC2] Rivest, R., "A Description of the RC2(r) Encryption
<draft-rivest-rc2desc-00.txt> Algorithm", RFC 2268, January 1998.
[RC4] R. Thayer and K. Kaukonen, A Stream Cipher Encryption [RC4] Thayer, R. and K. Kaukonen, A Stream Cipher Encryption
Algorithm, <draft-kaukonen-cipher-arcfour-01.txt>, July 1997. Algorithm, Work in Progress.
[RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key Cryptosystems," Obtaining Digital Signatures and Public-Key Cryptosystems,"
Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120-126. Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120-
126.
[RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782 [SCH] B. [RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782
Schneier. Applied Cryptography: Protocols, Algorithms, and Source
Code in C, Published by John Wiley & Sons, Inc. 1994.
[SHA] NIST FIPS PUB 180-1, "Secure Hash Standard," National [SCH] B. Schneier. Applied Cryptography: Protocols, Algorithms,
Institute of Standards and Technology, U.S. Department of Commerce, and Source Code in C, Published by John Wiley & Sons, Inc.
DRAFT, May 31, 1994. 1994.
[SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications [SHA] NIST FIPS PUB 180-1, "Secure Hash Standard," National
Corp., Feb 9, 1995. Institute of Standards and Technology, U.S. Department of
Commerce, Work in Progress, May 31, 1994.
[SSL3] A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol", [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications
Netscape Communications Corp., Nov 18, 1996. Corp., Feb 9, 1995.
[TCP] ISI for DARPA, RFC 793: Transport Control Protocol, September [SSL3] A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol",
1981. Netscape Communications Corp., Nov 18, 1996.
[TEL] J. Postel and J. Reynolds, RFC 854/5, May, 1993. [TCP] Postel, J., "Transmission Control Protocol," STD 7, RFC 793,
September 1981.
[X509] CCITT. Recommendation X.509: "The Directory - Authentication [TEL] Postel J., and J. Reynolds, "Telnet Protocol
Framework". 1988. Specifications", STD 8, RFC 854, May 1993.
[XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External Data [TEL] Postel J., and J. Reynolds, "Telnet Option Specifications",
Representation Standard, August 1995. STD 8, RFC 855, May 1993.
[X509] CCITT. Recommendation X.509: "The Directory - Authentication
Framework". 1988.
[XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External
Data Representation Standard, August 1995.
Credits Credits
Working Group Chair Win Treese
Open Market
Win Treese EMail: treese@openmarket.com
Open Market
treese@openmarket.com
Editors Editors
Christopher Allen Tim Dierks Christopher Allen Tim Dierks
Certicom Certicom Certicom Certicom
callen@certicom.com tdierks@certicom.com
Authors EMail: callen@certicom.com EMail: tdierks@certicom.com
Tim Dierks Philip L. Karlton Authors' Addresses
Certicom Netscape Communications
tdierks@certicom.com
Alan O. Freier Paul C. Kocher Tim Dierks Philip L. Karlton
Netscape Communications Independent Consultant Certicom Netscape Communications
freier@netscape.com pck@netcom.com
Other contributors EMail: tdierks@certicom.com
Alan O. Freier Paul C. Kocher
Netscape Communications Independent Consultant
Martin Abadi Robert Relyea EMail: freier@netscape.com EMail: pck@netcom.com
Digital Equipment Corporation Netscape Communications
ma@pa.dec.com relyea@netscape.com
Ran Canetti Jim Roskind Other contributors
IBM Watson Research Center Netscape Communications
canetti@watson.ibm.com jar@netscape.com
Taher Elgamal Micheal J. Sabin, Ph. D. Martin Abadi Robert Relyea
Securify Consulting Engineer Digital Equipment Corporation Netscape Communications
taher@pacbell.net msabin@netcom.com
Anil R. Gangolli Dan Simon EMail: ma@pa.dec.com EMail: relyea@netscape.com
Structured Arts Computing Corp. Microsoft
gangolli@structuredarts.com dansimon@microsoft.com
Kipp E.B. Hickman Tom Weinstein Ran Canetti Jim Roskind
Netscape Communications Netscape Communications IBM Watson Research Center Netscape Communications
kipp@netscape.com tomw@netscape.com
Hugo Krawczyk EMail: canetti@watson.ibm.com EMail: jar@netscape.com
IBM Watson Research Center
hugo@watson.ibm.com Taher Elgamal Micheal J. Sabin, Ph. D.
Securify Consulting Engineer
EMail: elgamal@securify.com EMail: msabin@netcom.com
Anil R. Gangolli Dan Simon
Structured Arts Computing Corp. Microsoft
EMail: gangolli@structuredarts.com EMail: dansimon@microsoft.com
Kipp E.B. Hickman Tom Weinstein
Netscape Communications Netscape Communications
EMail: kipp@netscape.com EMail: tomw@netscape.com
Hugo Krawczyk
IBM Watson Research Center
EMail: hugo@watson.ibm.com
Comments Comments
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