Authentication Against a Myopic Adversary

We consider keyless authentication for point-to-point communication in the presence of a myopic adversary. In particular, the adversary has access to a non-causal noisy version of the transmission and may use this knowledge to choose a channel state; the receiver is successful if it either decodes to the correct message or correctly detects adversarial interference. We show that, depending on the relationship between the channels to the legitimate receiver and the adversary, the authentication capacity is either zero or bounded above by the no-adversary capacity. To this end we introduce a novel channel property, termed U -overwritability, which allows the adversary to make its false message appear legitimate. We conjecture that if the capacity is nonzero, it is in fact equal to the no-adversary capacity, and show that this is true for a particular binary model.