  In this paper, we present a first step towards a formalisation of the Quantum
Key Distribution algorithm in Isabelle. We focus on the formalisation of the
main probabilistic argument why Bob cannot be certain about the key bit sent by
Alice before he does not have the chance to compare the chosen polarization
scheme. This means that any adversary Eve is in the same position as Bob and
cannot be certain about the transmitted keybits. We introduce the necessary
basic probability theory, present a graphical depiction of the protocol steps
and their probabilities, and finally how this is translated into a formal proof
of the security argument.
