Capacity Limits of Pseudorandom Channels in Deception Problems

Supraliminal channels were introduced in 1998 as a means to achieve public key exchange in the presence of an active warden. These channels have content visible to all principals-their content is not concealed or protected by a secret key-but are highly robust to an active warden. It is assumed that this high robustness allows the transmission of key exchange datagrams. In this paper, we provide a theoretical model for supraliminal channels as channels of random data with a constraint on their distribution. We present a surprising result that in such a model, vanishingly small tampering can indefinitely derail key exchange. Unlike traditional communication theory, the specific constraints of steganographic channels prevent the use of redundancy to achieve more reliable transmission, and can even make communication more fragile to an active adversary. This result requires a vigilant adversary, however, and we propose a protocol to increase the probability of successful key exchange in the event of a pause in the warden´s tampering.