  Proof of security of cryptographic protocols theoretically establishes the
strength of a protocol and the constraints under which it can perform, it does
not take into account the overall design of the protocol. In the past model
checking has been successfully applied to classical cryptographic protocols to
weed out design flaws which would have otherwise gone unnoticed. Quantum
cryptographic protocols differ from their classical counterparts, in their
ability to detect the presence of an eavesdropper. Although unconditional
security has been proven for both BB84 and B92 protocols, in this paper we show
that identifying an eavesdropper's presence is constrained on the number of
qubits exchanged. We first model the protocols in CQP and then explain the
mechanism by which we have translated this into a PRISM model. We mainly focus
on the protocols' ability to detect an active eavesdropper and the extent to
which an eavesdropper can retrieve the shared key without being detected by
either party. We then conclude by comparing the performance of the protocols.
