A Subliminal Channel in Secret Block Ciphers

In this paper we present the first general purpose subliminal channel that can be built into a secret symmetric cipher by a malicious designer. Subliminal channels traditionally exploit randomness that is used in probabilistic cryptosystems. In contrast, our channel is built into a deterministic block cipher, and thus it is based on a new principle. It is a broadcast channel that assumes that the sender and the receiver know the subliminal message ms (i.e., something derived from their common key). We show that the designer can expect to be able to read ms when O(|ms|log|ms|) plaintext/ciphertext pairs are obtained. Here |ms| is the length of ms in bits. We show how to turn the channel into a narrowcast channel using a deterministic asymmetric cipher and then present an application of the narrowcast channel. In this application, the secret block cipher securely and subliminally transmits the symmetric key of the sender and receiver to the malicious designer and confidentiality holds even when the cipher is made public.