ua nt - p h / 06 04 20 7 v 1 2 7 A pr 2 00 6 Quantum mutual information and the one - time pad

A one-time pad [1] is a cryptographic protocol in which communicators Alice and Bob initially have correlated random variables, collectively called the “key”, that are not correlated with any variables possessed by a potential eavesdropper Eve. (In most discussions, the key variables possessed by Alice and Bob are supposed to be perfectly correlated—e.g., they are identical copies of the same secret string of bits. We consider the more general case.) If the key variables are used only once, they allow Alice to send Bob a perfectly secret message over a public communication channel. The value of a key as a resource is the amount of information that can be sent secretly by its use.