Subliminal Channels

[Hal Finney]

Steganography, the art of hiding a secret message inside of an openly
readable one, can be thought of as a subliminal channel.  In the prisoner
example, the prisoners could have pre-arranged that, say, every 10th
character in the typed messages they exchange would be used to spell out
a secret message.  Or perhaps word or sentence lengths or spacings could
send a message.

In general, in any system where there is ambiguity, more than one way of
expressing a valid message, there is a subliminal channel.  Since DSS
signatures are apparently not unique for a given message (unlike, say,
RSA signatures as specified in the PKCS standards), they have such a
channel.

Hal