Remember those invisible ink kits from when you were a kid? You'd write a secret message that no one could see unless they had a black light or the decoder marker. The digital equivalent of invisible ink is steganography software, apps that embed files and data inside other files, hidden from everyone who doesn't know any better. You don't have to be a trained spy plotting international espionage to put steganography to good use. With some free tools for both the Mac and PC, you can embed secret information in image, PDF, HTML and MP3 files for fun or profit.

Unlike encryption, which obscures data in such a way that it's obvious someone's keeping something from listeners-in (and therefore heightens interest in that info), stego techniques offer no hint to the outsider that there's any private data contained within the visible file. Like hiding your valuables from burglars in an empty cereal box in your kitchen cabinet, steganography keeps the existence of the secret item from everyone but those in the know.

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