[1] CCITT Recommendation X.411 (1988), "Message Handling Systems: Message Transfer System: Abstract Service Definition and Procedures". [2] CCITT Recommendation X.509 (1988), "The Directory - Authentication Framework". [3] CCITT Recommendation X.520 (1988), "The Directory - Selected Attribute Types". [4] NIST Special Publication 500-183, "Stable Agreements for Open Systems Interconnection Protocols," Version 4, Edition 1, December 1990. [5] North American Directory Forum, "A Naming Scheme for c=US", RFC 1255, NADF, September 1991. [6] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures", RFC 1421, DEC, February 1993. [7] Balenson, D., "Privacy Enhancement for Internet Electronic Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423, TIS, February 1993. [8] Balaski, B., "Privacy Enhancement for Internet Electronic Mail: Part IV: Notary, Co-Issuer, CRL-Storing and CRL-Retrieving Services", RFC 1424, RSA Laboratories, February 1993. [9] North American Directory Forum, "NADF Standing Documents: A Brief Overview", RFC 1417, NADF, February 1993.