| The security and privacy of electronic communication has become an increasingly important issue due to its susceptibility to interventions. This dissertation addresses a series of issues regarding the war between the communicating parties and the adversary, which include how the communicating parties can achieve robust covert communication, what kind of adversary can eliminate the potentials for covert communication and under what circumstances the communicating parties can defeat such an adversary.;First, we developed two applications, a video application on Macintosh and a Walkie-talkie application on iPhone platform, which enable the users to robustly transmit secret messages in a censored environment by partially generating the perceptual content of the multimedia data. These two applications also demonstrate the feasibility of the supraliminal channel which is a theoretically sound approach proposed in 1998 as a solution for initializing key exchange.;Secondly, we built up a theoretical model named a coin flip channel for those robust communication channels discussed in the first part. Just as the name implies, the coin flip channel only carries a string of fair coin flips, namely, the transmitted data should be uniformly distributed. Further, we derived the theoretical bound on the pubic transmission limits over the coin flip channel and concluded the fundamental results as the Counter Deception Theorem which states that: without sharing a secret key in advance, it is generally impossible for the communicating parties to achieve covert communication if the adversary has the power to inject even a mild amount of noise. The counter deception theorem shows substantial pessimism on the potential that the communicating parties can communicate secretly in face of a capable adversary.;Finally, we presented a protocol for key exchange over a coin flip channel. This key exchange protocol shows that despite the pessimistic results of the counter deception theorem, under certain circumstances, it is still possible for the communicating parties to achieve covert communication without sharing a key in advance. |
