Abstract: Some defects of G.J.Simmons' subliminal-free protocol are discovered during our study. In this paper two narrow-band subliminal channels are given based on the protocol. One is the channel we have constructed based on harmless messages, and the other is the channel constructed by Y.Desmedt based on the component r of ElGamal signature (r,s). Then a sound discussion shows that it is impossible to construct a complete subliminal-free channel in digital signatures. At the same time the lowest transmission efficiency is discussed.

Key words: information hiding, digital signature, subliminal channel, subliminal-free protocol