Message passing by means of public key encryption is described in terms of doxastic dynamic logic. A secret message from $a$ to $b$ can have as effect that $b$ learns something new from $a$, but it can also cause a change in the real world, when the contents of the message forces $b$ to cease trusting $a$. As a tool for analysing secret message passing (and much else besides) we develop a framework that allows changes of states conditioned by beliefs about those states.

Deduction and Theorem Proving (acm I.2.3), Knowledge Representation Formalisms and Methods (acm I.2.4), Security and Protection (acm D.4.6)
Communication theory (msc 94A05), Authentication and secret sharing (msc 94A62), Logics of knowledge and belief (including belief change) (msc 03B42), None of the above, but in MSC2010 section 68Pxx (msc 68P99)
Information Systems [INS]
Standardization and Knowledge Transfer

Bleeker, A.M, & van Eijck, D.J.N. (2000). The epistemics of encryption. Information Systems [INS]. CWI.