  We give diagrammatic tools to reason about information flow within encrypted
communication. In particular, we are interested in deducing where information
flow (communication or otherwise) has taken place, and fully accounting for all
possible paths. The core mathematical concept is using a single categorical
diagram to model the underlying mathematics, the epistemic knowledge of the
participants, and (implicitly) the potential or actual communication between
participants. A key part of this is a `correctness' or `consistency' criterion
that ensures we accurately & fully account for the distinct routes by which
information may come to be known (i.e. communication and / or calculation). We
demonstrate how this formalism may be applied to answer questions about
communication scenarios where we have the partial information about the
participants and their interactions. Similarly, we show how to analyse the
consequences of changes to protocols or communications, and to enumerate the
distinct orders in which events may have occurred. We use various forms of
Diffie-Hellman key exchange as an illustration of these techniques. However,
they are entirely general; we illustrate in an appendix how other protocols
from non-commutative cryptography may be analysed in the same manner.
