In 1984, the CCITT (Consultative Committee on International Telegraphy and Telephony) approved the first version of the X.400 series of Recommendations [CCI88a], [CCI88b], [CCI88c]. The Recommendations defined a general purpose, store-and-forward, messaging service. In 1988, the CCITT updated the Recommendations to include, among other features, security services that protect messages against modification and disclosure, and allow communicating parties to authenticate their identities.
This chapter provides tutorial information about the 1988 X.400 security services. Section 11.1 introduces the chapter. Section 11.2 discusses cryptography as a tool to protect data transmitted over insecure channels. Beginning with section 11.3, all material is specific to the MHS (Message Handling System). Section 11.3 overviews the MHS. Section 11.4 describes its primary vulnerabilities. Section 11.5 describes the means by which security-relevant information is conveyed in the MHS. Section 11.6 details the X.400 security services that counter the vulnerabilities described in section 11.4, and section 11.7 concludes the chapter by discussing limitations in the 1988 X.400 security architecture. Additional reading on computer security can be found in [AMPH87], and additional reading on X.400 security can be found in [Ank92] and [CM89].