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.