[cryptography] ciphers with keys modifying control flow?

Jon Callas jon at callas.org
Mon Sep 27 21:11:28 EDT 2010

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.


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