Self-synchronized message randomization methods for subliminal channels

When one transmits a secret message sequence on a random number type subliminal channel, he/she has to convert the secret message sequence into a (practically) indistinguishable random number sequence first, and then embeds it on a carrier sequence. Otherwise the carrier sequence could be distinguished from one that contains no secret message. If others can distinguish whether a secret message sequence is embedded in the carrier sequence, the carrier sequence cannot be a subliminal channel. That is, a converter to convert any message sequence into a (practically) indistinguishable one is required. Moreover in many applications of subliminal channels, the deconverter corresponding to the converter should be self-synchronized with the converted sequence, because additional information to synchronize reduces the indistinguishability. Therefore, both (practical) indistinguishability and self-synchronization are required to the converter for subliminal channels. Vernum encryption can convert any message sequences into perfectly indistinguishable random number sequences. However the receivers cannot decode the message sequences from anywhere of the converted sequences without any knowledge of the synchronization. On the contrary, (EBC), CBC, CFB mode block ciphers and self-synchronizing stream ciphers can realize the self-synchronization. However, most of the output sequences can be distinguished from real or well-designed random number sequences by using the birthday paradox distinguishers we propose in this paper under some conditions.