On Some Methods for Unconditionally Secure Key Distribution and Broadcast Encryption

This paper provides an exposition of methods by which a trusted authority can distribute keys and/or broadcast a message over a network, so that each member of a privileged subset of users can compute a specified key or decrypt the broadcast message. Moreover, this is done in such a way that no coalition is able to recover any information on a key or broadcast message they are not supposed to know. The problems are studied using the tools of information theory, so the security provided is unconditional (i.e., not based on any computational assumption).We begin by surveying some useful schemes for key distribution that have been presented in the literature, giving background and examples (but not too many proofs). In particular, we look more closely at the attractive concept of key distribution patterns, and present a new method for making these schemes more efficient through the use of resilient functions. Then we present a general approach to the construction of broadcast schemes that combines key predistribution schemes with secret sharing schemes. We discuss the Fiat-Naor Broadcast Scheme, as well as other, new schemes that can be constructed using this approach.