Steganography
From Just Solve the File Format Problem
Steganography is a class of encryption in which the very fact that information exists is obscured. For example, an innocent-looking PNG image could contain a secret message encoded in its seemingly-random least-significant bits. Assuming the algorithm is sufficiently secure, it would be impossible to know whether the message exists or not, unless you know the secret key.
For a list of formats, see Encryption#Steganography.
