A supraliminal channel in a videoconferencing application

Unlike subliminal or steganographic channels, a supraliminal channel encodes information in the semantic content of cover data, generating innocent communication in a manner similar to mimic functions. These low-bitrate channels are robust to active wardens, and can be used with subliminal channels to achieve steganographic public key exchange. Automated generation of innocent-looking content, however, remains a difficult problem. Apple's iChat, a popular instant-messaging client and the default client on the Macintosh operating system, includes a video chat facility that allows the user to apply special effects such as replacing the user's background with a video file. We show how this can be used to implement a high-bitrate supraliminal channel, by embedding a computer animation engineered to communicate ciphertext by its pseudo-random behavior.