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*To*: [email protected], [email protected]*Subject*: really (?) undetectable crypto*From*: [email protected] (Peter M Allan)*Date*: Fri, 13 Sep 96 13:55:32 BST*Sender*: [email protected]

> [email protected] wrote on CP: > > Most everybody on the list is familiar with the technique of hiding > encrypted messages in the LSBs of image files. Personally, I would not > use such a technique because don't I believe it's really undetectable. I > assume, without proof, that the LSBs of images files have statistical > properties that are sufficiently different from encrypted data that a > clever person could determine whether or not an image file contained an > imbedded encrypted message. > Not to mention 7 out of 8 bits may reveal the image to be a library one your enemy has access to. The changes will betray the stego. Your own scanned snapshots may be safer from this point of view. > Fortunately, there are other steganographic techniques that, I believe, > are undetectable. The trick is to hide your encrypted bits in other > encrypted bits. > > trick #1) Let's say you want to send a short encrypted message via a > communications channel that only allows cleartext messages with optional > MD5 message hashes. You can construct cleartext messages, via > trial-and-error, such that the first 4 or 8 bits (or more, if you have the > time) of the MD5 hash match the first 4 or 8 bits of your encrypted > message. > > Since the bits in an MD5 message hash are presumably cryptographically > random, there should be no way to tell if some of the bits combine to make > an encrypted message. What about Walter making insignificant changes to the cleartext and replacing the hash with the new hash? Because you are using an unkeyed hash (and not a sig) he can do that and foul up the stegomessage (not that he'll yet be sure there is one). > trick #2) Let's say you are allowed to use 40 bit encryption, but nothing > stronger. As in trick #1, you can pre-compute plaintext messages such > that the first 4 or 8 of the bits in the output of the government-approved > 40 bit encrypted data match the first 4 or 8 bits of your hidden encrypted > message. > Walter can still play silly spooks with your stego if he breaks the 40-bit encryption. The cyphertext/plaintext ratio looks like getting really huge too. Your messages must all arrive, and retain the right order. -- Peter Allan [email protected]

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