Study On Subliminal Channels

A subliminal channel is a covert communication channel to send a message to an authorized receiver. This channel is constructed in the output crypto-data of an applied public-key cryptosystem such as digital signature, authentication, and so on. The message can not be discovered by any unauthorized receivers. It can be used in many practical applications as a representative information hiding technology. In this dissertation we make a study of subliminal channels that conclude the progress, the application, the model, the construction, the stego-key, the capacity, the security, the way to realize subliminal-freeness, and so on.The main results we have obtained in this dissertation are as follows:1. A general model of subliminal channels and some necessary conditions that the carrier cryptosystem must holds are given. A secret channel and an attack channel are first defined and some basic conclusions of the security are given. Two new parameters, the Embedding Rate and the Channel-Using Rate, that have large effects on the capacity and the security of subliminal channels are first defined.2. The subliminal channels available are classified according to the construction mechanism. A new broadband channel is designed in proxy signatures to reduce the risk of the original signer's signing capability being abused. A new narrowband channel with large capacity is designed based on a combination of the leaks of partial trapdoor key and the exhausting search. The reasons for the meaningless -ness in practical of the construction of a subliminal channel in authentication codes are analyzed. An abused system and a subliminal system are distinguished.3. The concept of stego-key exchange is first presented. Some conditions and a basic protocol of realizing it are first discussed. Then a known-secret stego-key exchange scheme and some unknown-secret stego-key exchange schemes are designed.4. The subliminal capacity is defined. A theoretical upper bound of the capacity and some factors that reduce the capacity are given. The effects of the Channel-Used Rate and the Embedding Rate on subliminal capacity are also given. The capacity of subliminal channels in ElGamal signatures is studied. The capacity of broadband channel and the optimal schemes in 18 general ElGamal signatures are analyzed. The relation between the capacity and the Embedding Rate of narrowband subliminal channels based on the exhausting search is studied. We also studied the capacity of Newton channel.5. The security conditions of one-time-used channels are given. The probabilistic detection of subliminal channels mainly under Bernoulli test with binomial distribution is studied. Some results of the security of many-time-used channels in hypothesis test model based on relative entropy are obtained.6. The schemes to avoid the subliminal channels are summarized and compared with each other. A conclusion is drawn that the subliminal channel introduced by the semantic randomness of the input harmless message of cryptosystem can not be completely avoided. A new scheme to generate ELGamal-type signatures that are free of any subliminal channels introduced by its session key is designed. The practicability of the subliminal-free protocols available is discussed.