The Additive Noise Channel with a Helper The additive noise channel is studied in the presence of a helper who observes the noise and can describe it to the receiver over a ratelimited noisefree bitpipe. It is shown that the capacity of this network is typically the sum of the capacity of the channel in the absence of the helper and the capacity of the bitpipe from the helper to the receiver. This holds for finitevariance stationary and ergodic noises under fairly general powerlike constraints on the transmitted signal. A helper that is only cognizant of the noise is thus as helpful as an omniscient helper that is cognizant of both the noise and the transmitted message. The achievability proof is based on xe2x80x9cflash helpingxe2x80x9d and requires no binning. Extensions to additivenoise multiaccess channels are also discussed.

On the Commitment Capacity of Unfair Noisy Channels. Noisy channels are a valuable resource from a cryptographic point of view. They can be used for exchanging secretkeys as well as realizing other cryptographic primitives such as commitment and oblivious transfer. To be really useful, noisy channels have to be consider in the scenario where a cheating party has some degree of control over the channel characteristics. Damgard et al. (EUROCRYPT 1999) proposed a more realistic model where such level of control is permitted to an adversary, the so called unfair noisy channels, and proved that they can be used to obtain commitment and oblivious transfer protocols. Given that noisy channels are a precious resource for cryptographic purposes, one important question is determining the optimal rate in which they can be used. The commitment capacity has already been determined for the cases of discrete memoryless channels and Gaussian channels. In this work we address the problem of determining the commitment capacity of unfair noisy channels. We compute a singleletter characterization of the commitment capacity of unfair noisy channels. In the case where an adversary has no control over the channel (the fair case) our capacity reduces to the wellknown capacity of a discrete memoryless binary symmetric channel.