SIGSALY
was the first secure voice encryption system for telephones.
It was invented and built by Bell Telephone Laboratories
in 1943. It had several technological "firsts" including
pulse code modulation for speech transmission, multilevel
frequency shift keying, and bandwidth compression.
SIGSALY used recordings of purely random noise for its security process. Both the sending and receiving station had copies of the acetate records and as one “encrypted” the voice, the other “decrypted.” Understandably, voice quality was quite poor, but it was secure. SIGSALY was not broken by the Axis powers.