  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 the channel state of an arbitrarily-varying channel between legitimate
users; the receiver is successful if it either decodes to the correct message
or correctly detects adversarial interference. We show that a channel condition
called U-overwritability, which allows the adversary to make its false message
appear legitimate and untampered with, is a sufficient condition for zero
authentication capacity. We present a useful way to compare adversarial
channels, and show that once an AVC becomes U-overwritable, it remains
U-overwritable for all "less myopic" adversaries. Finally, we show that
stochastic encoders are necessary for positive authentication capacity in some
cases, and examine in detail a binary adversarial channel that illustrates this
necessity. Namely, for this binary channel, we show that when the adversarial
channel is degraded with respect to the main channel between users, the
no-adversary capacity of the underlying channel is achievable with a
deterministic encoder. Otherwise, provided the channel to the adversary is not
perfect, a stochastic encoder is necessary for positive authentication
capacity; if such an encoder is allowed, the no-adversary capacity is again
achievable.
