Quantum cryptography: Security criteria reexamined

Abstract

The generally accepted security criteria for a whole class of protocols for quantum cryptography was analyzed. It was shown that the security criteria are flawed. The reason of the flaw was a standard assumption of the security analysis, namely that the so-called square-root measurement is optimal for eavesdropping purposes, is not true in general. The results show that there are rather large parameter regimes in which the optimal measurement extracts substantially more information that the square-root measurement.