Unconditional Authenticity and Privacy from an Arbitrarily Weak Secret

Unconditional cryptographic security cannot be generated simply from scratch, but must be based on some given primitive to start with (such as, most typically, a private key). Whether or not this implies that such a high level of security is necessarily impractical depends on how weak these basic primitives can be, and how realistic it is therefore to realize or find them in|classical or quantum|reality. A natural way of minimizing the required resources for information-theoretic security is to reduce the length of the private key. In this paper, we focus on the level of its secrecy instead and show that even if the communication channel is completely insecure, a shared string of which an arbitrarily large fraction is known to the adversary can be used for achieving fundamental cryptographic goals such as message authentication and encryption. More precisely, we give protocols|using such a weakly secret key|allowing for both the exchange of authenticated messages and the extraction of the key’s entire amount of privacy into a shorter virtually secret key. Our schemes, which are highly interactive, show the power of two-way communication in this context: Under the given conditions, the same objectives cannot be achieved by one-way communication only.