In 1984, the CCITT (Consultative Committee on International Telegraphy 
and Telephony) approved the first version of the X.400 series of
Recommendations.  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.  
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This paper provides tutorial information about the 1988 X.400 \cite{bib:x400}
security services.  Chapter \ref{x400-intro} introduces the paper.  
Chapter \ref{x400-overview} overviews the MHS (Message Handling System).
Chapter \ref{x400-vulnerabilities} describes its primary vulnerabilities.
Chapter \ref{x400-structures} describes the means by which 
security-relevant information is conveyed in the MHS.  Chapter 
\ref{x400-services} details the X.400 security services that counter the 
vulnerabilities described in Chapter \ref{x400-vulnerabilities}, and Chapter 
\ref{x400-limitations} concludes the paper by discussing limitations in the 
1988 X.400 security architecture.
