The quantum key distribution protocol with classical Bob, well known as semiquantum key distribution, has been proposed a few years back (Boyer et al., PRL 2007, PRA 2009). In this protocol Alice has the same capability as in the traditional BB84 protocol, but Bob is limited by the constraint that he can measure or prepare qubits only in
{∣0>,∣1>} basis and reflect any other qubit (or in other words, let it go undisturbed). In this scheme the qubits travel from Alice to Bob and then again from Bob to Alice. 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 be extract more information using our two-way strategy than what can be obtained by the direct application of one-way eavesdropping in case of BB84 protocol. Towards resisting this eavesdropping strategy, we modify the semiquantum scheme by incorporating a Hadamard gate at Bob's end.
Semiquantum key distribution (SQKD) allows two parties (Alice and Bob) to create a shared secret key, even if one of these parties (say, Alice) is classical. However, most SQKD protocols suffer from severe practical security problems when implemented using photons. The recently developed “Mirror protocol” [1] is an experimentally feasible SQKD protocol overcoming those drawbacks. The Mirror protocol was proven robust (namely, it was proven secure against a limited class of attacks including all noiseless attacks), but its security in case some noise is allowed (natural or due to eavesdropping) has not been proved yet. Here we prove security of the Mirror protocol against a wide class of quantum attacks (the “collective attacks”), and we evaluate the allowed noise threshold and the resulting key rate.