Steganography

In some situations, the mere existence of a message from Alice is enough to land her in trouble!
Human rights groups often have this problem

Alice first encrypts her message to Bob, and then "hides" that encrypted message in a more innocuous message, perhaps posted to a newsgroup.  This hiding in an innocuous message can be a number of things:
Modifications to colour in an image
Modifications to audio in a sound file
Modifications to wording, spacing, or punctuation in a text message
Use spam.  That flies around *everywhere*!

This is a lot like watermarking.