The Epistemics of Encryption Annette Bleeker, Jan van Eijck Message passing by means of public key encryption is described in terms of doxastic dynamic logic. A secret message from agent A to agent 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. Keyword(s): Epistemic logic; dynamic logic; epistemic actions; cryptographic communication.