Paul Syverson,
``Formal Semantics for Logics of Cryptographic Protocols,''
Proceedings of the Computer Security Foundations Workshop III
Franconia, NH June 1990. IEEE CS Press (Los Alamitos CA, 1990)

<blockquote>
A first order, epistemic logic for representing
and analyzing cryptographic protocols is presented along
with an associated model-theoretic semantics. The formal
language presented has distinct means to represent knowledge of an
individual word, e.g., the ability to recognize or produce a
decryption key, and propositional knowledge. A sample analysis of
a protocol is given.  
</blockquote>