The security of quantum key distribution is typically defined in terms of the mutual information between the distributed key $S$ and the outcome of an optimal measurement applied to the adversary’s system. We show that even if this so-called accessible information is small, the key $S$ might not be secure enough to be used in applications such as one-time pad encryption. This flaw is due to a locking property of the accessible information: one additional (physical) bit of information can increase the accessible information by more than one bit.

• Received 13 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.98.140502