Background

Information hiding systems have three different aspects that contend with one another: capacity, security and robustness.
[Chen et al., 2001]      
Watermarking: robustness.
Steganography: security and capacity.
Often implies that hidden information is fragile.

Security:
Classical steganography relies on secrecy of encoding system.
Tattoo on a shaved head (440 B.C.),
Last word in every sentence,
Least-significant bits in an image.

Modern steganography attempts to be detectable only if secret information is known.
A secret key, etc...
Similar to Kerckhoff's Principle in cryptography.