On Sep 26, 2010, at 6:14 PM, Steven Bellovin wrote:
> Does anyone know of any ciphers where bits of keys modify the control path,
> rather than just data operations? Yes, I know that that's a slippery
> concept, since ultimately things like addition and multiplication can be
> implemented with loops in the hardware or firmware. I also suspect that it's
> potentially dangerous, since it might create very hard-to-spot classes of
> weak keys. The closest I can think of is SIGABA, where some of the keying
> controlled the stepping of the other rotors.
If you remember (or even read) Dan Brown's mid-late-'90s novel, "Digital
Fortress," in which the MacGuffin was an unbreakable cipher, I was amused when
I read it that Brown blurred the difference between code and data. The keys in
Brown's cipher were both data and self-modifying instructions in some virtual
machine. It didn't make sense in the way that software actually works, but this
made the cipher some weird cross between crypto and a polymorphic virus. It was
plain in the description that he'd gotten a little knowledge about each and
combined them to create a dangerous thing.
There have also been LFSR-type things that used some bits to control stepping
of other things, a la SIGABA, but I think that Digital Fortress is the closest
thing to what you want.
Jon
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