Practical unconditionally secure two-channel message authentication

We investigate unconditional security for message authentication protocols that are designed using two-channel cryptography. (Two-channel cryptography employs a broadband, insecure wireless channel and an authenticated, narrow-band manual channel at the same time.) We study both noninteractive message authentication protocols (NIMAPs) and interactive message authentication protocols (IMAPs) in this setting. First, 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 and Safavi-Naini, which was based on probability distribution arguments. We also prove a new result which holds in a weakened attack model. Further, we propose a generalization of an unconditionally secure 3-round IMAP due to Naor, Segev and Smith. The IMAP is based on two \epsilon-\Delta universal hash families. 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. Finally, a variation of the 3-round IMAP is presented, in which only one hash family is required.