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Solution Against Cold Boot Attack In the Making

Posted by kdawson on from the warm-it-by-the-fire dept.

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."

12 of 260 comments (clear)

  1. Freeze the CPU by despe666 · · Score: 5, Insightful

    Good idea, until they figure out how to cold boot the CPU as well.

    1. Re:Freeze the CPU by msgmonkey · · Score: 4, Interesting

      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.

    2. Re:Freeze the CPU by Anonymous Coward · · Score: 5, Informative

      You need to understand that there are different types of RAM. The main memory, that of which you have gigabytes, is DRAM. CPU caches are SRAM.
      DRAM is, essentially, a tiny capacitor that is regularly recharged. If you cool it down, it doesn't lose its charge as fast, so you can read it even after power loss.
      SRAM works differently. The data is stored by a few transistors wired together in a way so they can maintain a specific set state even when the external input goes away. There are no capacitors involved here, so once the supply voltage drops, the data is lost.

    3. Re:Freeze the CPU by bperkins · · Score: 4, Interesting

      Most cache is implemented as a flop, which loses its state on power down almost immediately. I don't think it's possible.

    4. Re:Freeze the CPU by ion.simon.c · · Score: 4, Insightful

      Thus, if the attacker has physical access to your box, you're screwed!

    5. Re:Freeze the CPU by bperkins · · Score: 4, Insightful

      Sorry, flop == flip-flop.

    6. Re:Freeze the CPU by SlashWombat · · Score: 4, Informative

      Carefully repowering SRAM can maintain the contents. I have seen SRAM come up with essentially 99% of the contents still intact after the SRAM had been powered down for over a week. I guess that once powered up, the SRAM has a preference to come back the way it was before powerdown.

      Or perhaps the slight residual voltage kept the SRAM contents intact. (Even though it was probably less than one tenth of a volt.) SRAM draws very little current when the voltages are reduced. Thus the power rails can maintain some small voltage for a very long time. .

    7. Re:Freeze the CPU by SteelFist · · Score: 5, Funny

      You mean like the sandals? Now I'm really confused...

  2. Great! by Anonymous Coward · · Score: 5, Funny

    Man I've been waiting for this! Lately the risk of a cold boot attack has really scared me, it's to the point where I don't even turn my computer on anymore!

  3. Adds another layer to hardware solutions? by Zergwyn · · Score: 4, Interesting

    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.

  4. Re:Write a summary that's useful, kthx. by Anonymous Coward · · Score: 4, Informative

    Not to mention, this is posted on the internet. Select the text you don't know about, right click and choose search google for "text". Firefox automagically opens a search on the topic in a new tab, and chances are the Wikipedia article is one of the top five results which should be a good enough starting point to see if the topic is of any interest to you. Magically, the old style of handholding journalism where authors are forced to assume the reader has the education of a 5th grader goes away, and society can actually advance further than the lowest common denominator.

  5. Re:Write a summary that's useful, kthx. by freedumb2000 · · Score: 4, Insightful

    Don't be arrogant and put the blame on the reader. It's called journalistic writing and typing a good summary does take a bit of care. Adhering to some basic writing principles, like the inverted pyramid, would go a long way even for a lowly summary writer/story submiter: http://en.wikipedia.org/wiki/Inverted_pyramid