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Solution Against Cold Boot Attack In the Making
Bubba writes "I just discovered this blog: Frozen Cache. It describes a concept for preventing cold boot attacks by saving the encryption key in the CPU cache. It is claimed that by disabling the CPU cache the key will remain in cache and won't be written to memory. The blog says they're working on a proof-of-concept implementation for Linux. Could this really turn out to be a working solution?" Update: 01/19 20:26 GMT by KD : Jacob Appelbaum, one of the authors of the cold boot attack paper, wrote in with this comment: "It's not a solution. It simply seeks to make it more obscure but an attacker would certainly still be able to pull off the attack. From what is on that blog, there's still a full keyschedule in memory at this time. This is how we reconstruct the key, the redundant information in memory; it's not just the 128/256 bit key itself. For older methods, they needed the actual specific key bits but we don't need them because we recreate them. Basically, the CPU is acting as a ghetto crypto co-processer. Emphasis on ghetto. It's a nice suggestion but the devil is in the details and sadly the details in this case aren't really up to snuff. It's a bogus solution."
Or the converse, I suppose (hardware solutions can add another layer to this). This looks like some very interesting work, and may have more applicability in general beyond this one scenario. I'm certainly looking forward to following their implementation as it comes along. But with that said, if this attack was a serious concern for a given entity there seem to be some obvious potential hardware solutions. The attack essentially depends on being able to shutdown the computer but keep the memory cold enough that the randomization time is slowed down tremendously, giving enough time to perform a dump of the contents onto another system for further analysis. Therefore, it can be prevented by, for example, having electric heater units surrounding the memory connected to a dedicated capacitor bank and temperature sensor, as well as a sensor to detect if someone tries for force open the machine (intrusion alarm). Then the system can perform a scram shutdown (or if it is just shutdown normally), and the heaters can assure that the memory is kept hot for a couple of seconds afterwards even in the face of attempted cooling. It only needs to manage it very briefly and then all the contents are scrambled. Other similar methods (maybe a really micro EMP inside a shield memory space) would be possible to, but basically they just need to deny an attacker for a very short amount of time or ensure entropy in the RAM and then the attack is useless.
Ultimately a dedicated hardware secure key store would be better and easier to integrate across all systems, and this more software solution of course has the massive advantage of being able to run for free on existing hardware. But the above could at least be retrofitted on nearly anything, and while it is more esoteric, then again so is the attack since it requires physical access.
Although marked as Funny, you could quite easily open the PC take out the BIOS ROM whilst the machine is running (the BIOS is shadowed in normal operation), stick a new custom one in that probes the cache and do a hard reset.
Most cache is implemented as a flop, which loses its state on power down almost immediately. I don't think it's possible.