Abstract : Many encryption methods involving chaotic dynamics have been proposed in the literature since the early 90's. Most of them mask the confidential information being transmitted through an insecure channel, with a chaotic analog or digital sequence. The recovering of the original information usually calls for reproducing, at the receiver side, the same chaotic signal as at the transmitter side. The synchronization mechanism of the two chaotic signals is known as chaos synchronization. In this communication, a connection between chaotic and conventional encryption is established via the control-theoretical condition of flatness, with special emphasis on one of the most attractive schemes, namely, message-embedding. The main conclusion can be stated as follows: the message-embedded cryptosystem is equivalent to a conventional self-synchronizing stream cipher under the flatness condition. It follows that both architectures have the same level of security
