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Phoenix666 writes:

A mimic function changes a file A so it assumes the statistical properties of another file B. That is, if p(t,A) is the probability of some substring t occuring in A, then a mimic function f, recodes A so that p(t,f(A)) approximates p(t,B) for all strings t of length less than some n. This paper describes the algorithm for computing mimic functions and compares the algorithm with its functional inverse, Huffman coding. It also provides a description of more robust mimic functions which can be defined using context-free grammars.

In his short story, "The Purloined Letter", Edgar Allan Poe describes a search by the police for an incriminating letter. The police ransack the house and pry open anything that might be hiding it, but they cannot find it. They look for hidden compartments, poke in mattresses and search for secret hiding spaces with no success. The detective, C. Auguste Dupin, goes to the house and finds the letter hidden in a different envelope in plain sight. He says, "But the more I reflected upon the daring, dashing and discriminating ingenuity, ... the more satisfied I became that, to conceal this letter, the Minister had resorted to the comprehensive and sagacious expedient of not attempting to conceal it at all."

In many ways, the practical cryptographer faces the same problem. Messages need to get from one place to another without being read. A traditional cryptographer tries to guarantee the letter's security by sealing the message in a mathematical safe and shipping the safe. There is no attempt made to hide the fact that it is a letter at all. The cryptanalyst attacking the message may or may not be able to break the code, but he has little problem finding and identifying the carrier.

Many of the histories written about the cryptography community, however contain stories of how the analysis of the message traffic alone lead to intelligence coups. Mimic functions hide the identity of a text by recoding a file so its statistical profile approximates the statistical profile of another file. They can convert any file to be statistically identical to, for instance, the contents of the USENET newsgroups like rec.humor or the classified section of the Sunday New York Times. Their contribution to security is largely founded upon the assumption that the explosion of information traffic makes it impossible for humans to read everything. Anyone watching must use computers outfitted with statistical profiles to weed the interesting data from the mundane.

Original Submission