Information hiding is a new area in the research of information security.It is significantly distinguished from encryption.Subliminal channel existing in some digital signature schemes is a kind of secure channel used for transmitting hidden information.But the information transmitted through this subliminal channel is very limited.Supraliminal channel is a newly presented secure channel used in the case that the receiver and the transmitter share no key ahead of their communication.Supraliminal channel can avoid man-in-the-middle attack.Supraliminal information may be treated as public key for the protection of the subsequent hidden information.Information theoretic analyses are made for the security of the two sorts of information hiding technique.Some security conditions are given.