Seaman and Pence (1995) further developed the checksum concept and wrote a detailed proposal describing a FITS convention for using the CHECKSUM and DATASUM keywords to verify the contents of FITS files. This original proposal is available in html and postscript format.
Recently, Seaman, Pence, and Rots (2002) have produced a more concise checksum proposal which preserves most of the technical details and discussion of the original proposal. The latest 23 May 2002 version of this proposal is available here in html and postscript format.