A distributed multipurpose mail guard

We describe a mechanism for incorporating a mail guard mechanism together with automatic, mandatory, and fully transparent digital signatures and encryption for message traffic embedded into the operating system of individual network nodes. By intercepting all inbound and outbound network traffic and analyzing for pertinent information using generalized Buchi automata, the guard mechanism can enforce the application of (centralized) mail security policies without requiring any support from mail clients. An implementation based on modular modifications to the Microsoft Windows NT/2000/XP family of operating systems and OpenPGP-based messaging is described.