Eavesdropping in semiquantum key distribution protocol

• Boyer et al. introduced the semiquantum key distribution protocol with classical Bob and showed its robustness.
• However, any analysis relating the amount of information leakage to the eavesdropper and the disturbance caused by her was open.
• We analyze this scheme and explicitly derive eavesdropperʼs advantage as a function of the disturbance.
• Our proposed eavesdropping strategy exploits both the directions of communication between the two parties.
• Our two-way eavesdropping extracts more information on the secret bits than one-way application of the strategy that worked on BB84.

In semiquantum key-distribution (Boyer et al.) Alice has the same capability as in BB84 protocol, but Bob can measure and prepare qubits only in {|0〉,|1〉}{|0〉,|1〉} basis and reflect any other qubit. We study an eavesdropping strategy on this scheme that listens to the channel in both the directions. With the same level of disturbance induced in the channel, Eve can extract more information using our two-way strategy than what can be obtained by the direct application of one-way eavesdropping in BB84.