author : Peter Wayner

If you have digital data you want to keep secret, you can encrypt it. But that can just signal the presence of a valuable secret, and encourage people to try to crack the code. Steganography, or 'hidden writing', is a way of hiding your data in other, innocent looking data, so that no-one even suspects it's there. I'd heard of it before, in the context of hiding data in the least significant bits of an image, but this fascinating book told me more about the subject than I had dreamed existed.

Each chapter is broken into three parts: a 'humorous allegory', a general introduction, and the technical meat. I found the allegories sometimes humorous, sometimes bizarre, and sometimes just downright incomprehensible (if there is a hidden message in them, I failed to find it). But the general introductions are excellent: I particularly liked the intuitive explanations of how certain kinds of cryptography work. And the technical meat gives enough detail for people to see how to go about implementing some of the ideas, and pointers into the literature and to the Web for more information.

And for any governments who are worried by steganography, and might be tempted to mandate the use of lossy compression techniques only: don't bother. Someone has already worked out how to hide data in JPEG images!

• Encryption as a means of white noise generation
• Hiding data as 'errors', and using error correcting codes
• Sharing data amongst n people, so that k of them are needed to reconstruct it.
• Deniability: hiding your data in several images, some of which are not yours, so that it is impossible to tell who hid it.
• Data compression, and using decompression of white noise to produce patterns
• Mimicking the frequencies in ordinary text
• Using grammars to hiding data in automatically generated but 'plausible' text
• Hiding data using reversible Turing machines (the data is the input, the hidden result is the output)
• Hiding data in the low bits of images and in sound files; which bits to use
• Hiding data in lossily compressed JPEG images
• Hiding data in digital signatures
• Anonymous remailers
• 'Dining cryptographers': broadcasting messages so the sender is unknown