A Supraliminal Channel in a Wireless Phone Application

Supraliminal channels are robust public channels based on multimedia content, allowing innocuous key exchange in the presence of an active warden. One approach to supraliminal communication embeds bits in the pseudo-random state used to produce computer-generated special effects in hybrid media such as a videoconferencing session. In this paper we describe an application for wireless phones whose audio effects betray pseudo-random state and hence potential key information for an innocuous exchange protocol. Implementation of a walkie-talkie application for the Apple iPhone shows that hundreds of bits can be reliably transmitted per effect.