On Mar 19, 2008, at 6:56 PM, Steven M. Bellovin wrote:
I've been thinking about similar issues.  It seems to me that just
destroying the key schedule is a big help -- enough bits will change in
the key that data recovery using just the damaged key is hard, per
comments in the paper itself.

It is. That's something everyone should consider doing. However, I was struck by the decay curves shown in the Cold Boot paper. The memory decays in an S-curve. Interestingly, both the smoothest S-curve and the sharpest were in the most recent equipment.

However, this suggests that for a relatively small object (like a 256- bit key) is apt to see little damage. If you followed the strategy of checking for single-bit errors, then double-bit, then triple-bit, I hypothesize that this simple strategy would be productive, because of that curve.

(I also have a few hypotheses on which bits will go first. I hypothesize that a high-power bit surrounded by low-power ones will go first, and a low-power bit amongst high-power ones will go last. I also hypothesize that a large random area is reasonably likely to get an early single-bit error. My rationale is that the area as a whole is going to have relatively high power 'consumption' because it is random, but the random area is going to have local artifacts that will hasten a local failure. Assuming that 1 is high-power and 0 is low- power, you expect to see a bitstring of 00100 or 0001000 relatively often in a blob of 32kbits (4KB) or 64kbits (8KB), and those lonely ones will have a lot of stress on them.)

Despite that my hypotheses are only that, and I have no experimental data, I think that using a large block cipher mode like EME to induce a pseudo-random, maximally-fragile bit region is an excellent mitigation strategy.

Now all we need is someone to do the work and write the paper.


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