  Multiphoton state in quantum cryptography decreases its security. Key
disclosing with universal quantum cloning machine (UQCM) is considered in
explicit manner. Although UQCM cannot make perfect clones, there is some
invariant quantity between the original photon and the imperfect clones. The
invariant quantity, the direction of Stokes parameters, tells us the auxiliary
information leading into key information. The attack, then, corresponds to some
kind of quantum non-demolition measurement. Its application to recent
high-performance quantum cryptography, Y-00 protocol, is also studied.
