There is only one safe place for private data and messages: the place where nobody looks for it. A file encrypted with algorithms like PGP are not readable, but everybody knows that there is something hidden. Wouldn't it be nice, if everyone could open your encrypted files, and see noisy photos of some old friends instead of your private data? They surely wouldn't look for pieces of encrypted messages in the pixel values.
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http://en.wikipedia.org/wiki/Steganography