Steganography is the science of sending messages hidden in harmless
communications over a public channel so that an adversary eavesdropping 
on the channel cannot even detect the presence of the hidden messages. In this
paper, we formalize and propose a steganographic signature scheme. We define the
security condition of steganographic signature,
the steganographic security and the unforgeability. 
We construct a steganographic signature scheme, and we show that our
proposed steganographic signature scheme with the extended Schnorr signature
scheme is steganographically secure and unforgeable.