Restricted attacks on semi-quantum key distribution protocols

In this paper, we investigate single state, semi-quantum key distribution protocols. These are protocols whereby one party is limited to measuring only in the computational basis, while the other, though capable of measuring in both computational and Hadamard bases, is limited to preparing and sending only a single, publicly known qubit state. Such protocols rely necessarily on a two-way quantum communication channel making their security analysis difficult. However we will show that, for single state protocols, we need only consider a restricted attack operation by Eve. We will also describe a new single state protocol which permits “reflections” to carry information and use our results concerning restricted attacks to show its robustness.