Message encryption in a X.400 message handling system

The authors describe a message security mechanism and its implementation on an experimental X.400 message handling system (MHS). An X.400 MHS usually consists of a number of heterogeneous machines communicating in an open system interconnection (OSI) environment which may be subjected to a security threat. A model is set up to identify various levels of security in an MHS network. The security measures are provided at the network level, that is, between the originating and the recipient message transfer agents (MTAs). A decentralized public-key based message encryption algorithm is used to eliminate the requirement of a key distribution center. A set of protocol procedures is defined to enable the change of encryption key at an MTA. A windowing concept is applied to encryption keys to reduce the potential conflict of key changes due to nonuniform message delay through the network