A progress report on subliminal-free channels

Subliminal channels are closely related to covert channelsand are used to hide secret information. They abuse the communications resource. Subliminal channels can be introduced in many cryptographicsystems, and exploit the inherent randomness of the systems. For example, secret information can be hidden in the randomness of the authenticators of an authentication system. Similarly secret information can be hidden in the randomness (of the prover or verifier) of both zeroknowledge proof systems and signature systems. To establish a subliminal channel the cryptosystem is abused, that is, used in a different way and for a different purpose than intended by its designer. A particularly obnoxious type of subliminal channel may be activated by abortive halting. For state-of-the-art security, it may be desirable to detect, and if possible prevent, subliminal channels. In this paper we address the problem of whether it is possible to develop (and if so, how) appropriate techniquesfor detecting or preventing the use of such channels. Several such techniques have already been proposed in the literature, and are suitable for many systems. We review these. We also consider recent developments, in particular with regards to the formal security requirements and their impact on research.