Active sensing and side channels of information leakage in quantum cryptography

Various Trojan-horse attacks on quantum cryptography systems are systematically considered: an attack with passive detection of side radiation, an attack with active sensing, as well as joint attacks on states in the quantum channel, side and reflected radiation. It is shown that the most effective attack is an attack with the measurement of the number of photons in the reflected states and further joint measurement of information and reflected quantum states. Explicit estimates are obtained for the intensity of the reflected quantum states at which secret keys can be transmitted to a given length.