Malicious Cryptography: Exposing Cryptovirology

Chapter 10

Subliminal Channels

Subliminal channels form the cornerstone of modern kleptography. They can be utilized in the payloads of cryptoviruses and cryptotrojans to implement robust backdoors in cryptographic algorithms. Subliminal channels were originally investigated to demonstrate a weakness in a nuclear arms control verification protocol. In this regard subliminal channels demonstrated a plausibility result: given a carefully selected cryptosystem, a sneaky Trojan horse program could be developed for that cryptosystem that leaks which missile silos contain nuclear missiles. The applications of subliminal channels grew to encompass devastating smart-card insider threats. The classic abuse of subliminal channels is in the Prisoner's Problem that was formulated by Gus Simmons. This problem is described in this chapter along with numerous known subliminal channels.

On the surface it would seem that whenever a subliminal channel exists in a cryptosystem, this is a bad thing. However, ironically enough, this is not the case. It has been shown that this communication channel can sometimes be used in a non-subversive and publicly advertisable way. The chapter concludes with a description of how a subliminal channel can be used to help solve the key escrow problem. In a nutshell it is often possible to utilize a subliminal channel to securely transmit a private key to the key escrow ...