The epistemics of encryption

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.