Practical unconditionally secure two-channel message authentication

We investigate unconditional security for message authentication protocols that are designed using two-channel cryptography. We look at both noninteractive message authentication protocols (NIMAPs) and interactive message authentication protocols (IMAPs). We provide a new proof of nonexistence of nontrivial unconditionally secure NIMAPs. This proof consists of a combinatorial counting argument and is much shorter than the previous proof by Wang et al., which was based on probability distribution arguments. Further, we propose a generalization of an unconditionally secure 3-round IMAP due to Naor, Segev and Smith. With a careful choice of parameters, our scheme improves that of Naor et al. Our scheme is very close to optimal for most parameter situations of practical interest.