On the Channel Capacity of Narrow-Band Subliminal Channels

Subliminal channels, discovered by Simmons, yield a possibility to transmit covert messages by embedding them in cryptographic digital data, such as (EC)DSA signatures. The embedded messages are used for transmitting important information, or as watermarks or imprints of the data. Anyone can use or abuse these channels very easily because most cryptographic digital data widely used in the Internet is not subliminal-free. For example, DSA signatures are not subliminal-free and they are widely used in various applications, such as PGP, SSH2 and so on. It is very important to evaluate the ability of such channels. In this paper, we evaluate the channel capacity of narrow-band subliminal channels where a transmitter tries only the limited number of input values. Then, we apply this result to a practical model where carriers are transmitted one after another successively. (A carrier denotes data in which a covert message is embedded.) We show, under this model, memories can be used to increase the channel capacity, and then we compare their channel capacities. In addition, we show it is possible to reduce the computational complexity of establishing a narrow-band subliminal channel.

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