EAVESDROPPING STRATEGIES AND REJECTED-DATA PROTOCOLS IN QUANTUM CRYPTOGRAPHY

We show that rejected-data protocols for quantum cryptography are secure against generalized intercept/resend strategies of an eavesdropper provided that the legitimate users of the communication channel use at least three bases. We discuss the connection between this result and the recently-developed protocol based on violations of a suitably-constructed Bell-type inequality for single particles. We also give a new estimate of the probability that an eavesdropper remains undetected under the original protocol and thereby show that the optimal strategies available to an eavesdropper are further limited to those which randomize the errors between the sub-ensembles of data. This result also has implications for the way in which the legitimate users of the channel choose their test data.