A semiquantum key distribution (SQKD) protocol makes it possible for a
quantum party and a classical party to generate a secret shared key. However,
many existing SQKD protocols are not experimentally feasible in a secure way
using current technology. An experimentally feasible SQKD protocol, "classical
Alice with a controllable mirror" (the "Mirror protocol"), has recently been
presented and proved completely robust, but it is more complicated than other
SQKD protocols. Here we prove a simpler variant of the Mirror protocol (the
"simplified Mirror protocol") to be completely non-robust by presenting two
possible attacks against it. Our results show that the complexity of the Mirror
protocol is at least partly necessary for achieving robustness.Comment: 9 page
