All cryptosystems currently m use are symmetrm m the sense that they require the transmitter and receiver to share, m secret, either the same pmce of reformation (key) or one of a paLr of related keys easdy computed from each other, the key is used m the encryption process to introduce uncertainty to an unauthorized receiver. Not only is an

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 rst, 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, (ECB), 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. In this paper, we design some pairs of converters and deconverters that satisfy both (practical) indistinguishability and self-synchronization.

Other researchers have studied the feasibility of a brute force attack on DES using several known plaintexts. In practice, known plaintext / ciphertext pairs may not be readily available, but statistical information about similar plaintexts is much more easily aquired. Accordingly, we design a statistical plaintext recognizer suitable for use in a ciphertext-only key search machine. Software simulations indicate that this design gives a powerful attack on the encryption of low-entropy data.