With rapid development and wide use (especially in the area of E-commerce)
of the Internet, secure communication is going to be a more and more important
issue. In the development of security protocols, formal methods are used to
verify various properties of the underlying protocol without concerning its
mathematical and algorithmic details. Keeping SET (Secure Electronic
Transactions) protocol as our application background, we propose a set of
axioms and rules that captures the Non-monotonic and Dynamic natures of
security protocols with encrypted messages. Such a logic can be implemented in
deduction environments such as DFKI's VSE.