  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" [Boyer, Katz, Liss, and Mor, Phys. Rev. A 96, 062335 (2017)] 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.
