Background

Steganographic systems change the cover medium to embed information and may leave detectable traces within a medium's characteristic.
Secrecy of the information is not degraded, but its hidden nature is revealed, defeating purpose of steganography.

Information theoretic model for security against passive eavesdropper:
Adversary has complete knowledge of encoding system but does not know the secret key.
System is perfectly secure if no decision rules exists that performs better than random guessing.
[Cachin., 2000]      

Necessary conditions in practice:
Secret key and cover medium unknown to adversary.