Weekly Edition
Return to the Security page
| |||||||||||||
Cold Reboot Attacks on Disk Encryption
Ed Felten's Freedom to Tinker weblog has a report on research he and his colleagues have done on subverting whole disk encryption by reading the keys from RAM after the machine has been power-cycled. "The root of the problem lies in an unexpected property of todays DRAM memories. DRAMs are the main memory chips used to store data while the system is running. Virtually everybody, including experts, will tell you that DRAM contents are lost when you turn off the power. But this isn't so. Our research shows that data in DRAM actually fades out gradually over a period of seconds to minutes, enabling an attacker to read the full contents of memory by cutting power and then rebooting into a malicious operating system."
(Log in to post comments)
Cold Reboot Attacks, indeed Posted Feb 21, 2008 15:51 UTC (Thu) by abatters (✭ supporter ✭, #6932) [Link] > Cool the chips in liquid nitrogen (-196 °C) and they hold their state for hours at least, without any power. Just put the chips back into a machine and you can read out their contents. Now *that* is a cold reboot attack.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:02 UTC (Thu) by flewellyn (subscriber, #5047) [Link] Well, that's...unsettling. What to do about this problem? The only idea that comes to mind is programming the system firmware to "wipe" all system RAM on startup, overwriting the contents of RAM with zeroes prior to loading any operating system. I'm not sure if that would help, though.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:11 UTC (Thu) by johnkarp (subscriber, #39285) [Link] Many system firmwares have the option to do a full memory test, which presumably overwrites RAM at some point. You'd have to set a BIOS password to keep them from skipping it. Though newer boards seem to have that option less often in the interest of faster boots.
Cold Reboot Attacks on Disk Encryption Posted Feb 25, 2008 7:04 UTC (Mon) by drag (subscriber, #31333) [Link] The only solution to this sort of attack is to have the keys always in encrypted form in-memory.`Then when the system needs to have the key it can be decrypted and accessed quickly in CPU cache. The sort of memory used in CPU cache is different then the sort that is used in main system memory. So hopefull that cpu cache, besides being much faster, does not have the same properties of regular Dram. Or maybe have a built-in hardware tripwire that would 'zap' the ram in case of sudden power loss. Nothing that would destroy the ram, but enough to flip all the bits back to zero or something like that. I expect that the only total solution would be change the design of the hardware like that. Hopefully something that could be built-into the ram so that you would not have to modify the design of the motherboard.
Cold Reboot Attacks on Disk Encryption Posted Feb 29, 2008 22:19 UTC (Fri) by giraffedata (subscriber, #1954) [Link] The only solution to this sort of attack is to have the keys always in encrypted form in-memory.`Then when the system needs to have the key it can be decrypted and accessed quickly in CPU cache. Where do you keep the key for the key? And how does a program put something in the CPU cache without it also being in memory?
Cold Reboot Attacks on Disk Encryption Posted Mar 2, 2008 5:27 UTC (Sun) by kevinbsmith (guest, #4778) [Link] "Where do you keep the key for the key?" Keep in mind that "Security" is a spectrum, not a boolean. I have heard that it is pretty quick (and effective) to simply test every x-bit value that exists in RAM, and see if it happens to be the key you are seeking. A billion tries don't take too long. But if the in-RAM key were an encrypted version, and the key for the key were stored at some other random location in RAM, then you just turned the search from n to n^2. Not a bad trick if n is measured in gigas. Another option would be to split the real key up into several pieces, and store them in disparate locations. I haven't thought that one through enough to decide if it would be better or worse than the key-for-a-key option. Nor how many pieces would be optimal. Seems promising based on a few seconds of thought. Is either of these approaches Perfect? Absolutely not. Are they enough of an improvement to be worth doing? I would guess so, but would be interested to hear other opinions. If it turned out to be effective against one specific, popular, real-world attack, and the cost is low, sure. Looking a bit deeper, an attacker could look at memory patterns (and/or source code) to reduce the search space. But at least that would require the attacker to create a custom attack rather than using an off-the-shelf cracking script. Again, it's about raising the bar, when the costs of doing so are lower than the benefits.
Cold Reboot Attacks on Disk Encryption Posted Mar 3, 2008 6:40 UTC (Mon) by njs (guest, #40338) [Link] Searching all of memory for a given bitstring is indeed very quick -- a few seconds. But, err, if you already know what the bitstring for the key is, why are you searching for it? What you'd really be searching for would be the data structure that's holding the key -- the OS has some way to figure out which random-looking bitstring in those gigabytes of memory is really the key, so you can too, by finding the right data structures and chasing the right pointers. AFAICT that means that the techniques you're suggesting won't work -- if the key is split into multiple pieces, then don't look for the pieces, look for the data structure that lists off where all of the pieces are so they can be found again by the OS.
Cold Reboot Attacks on Disk Encryption Posted Mar 8, 2008 13:55 UTC (Sat) by kevinbsmith (guest, #4778) [Link] You missed the point. If you know that a key exists somewhere in RAM, it is fast and simple to read a key's worth of data starting at every byte in the system, and attempt a decode of every one until you find one that works. Brute force with one giga of attempts. No knowledge of OS, language, data structures, or app code is required, aside from knowing the encryption algorithm and key size (which is usually easy to find). Dead simple. And quite effective. Splitting the key, or having an encrypted key with its key elsewhere in RAM would foil that attack. At that point, the cracker would have to know something about the specific app and craft a custom attack, which makes the attack quite a lot more difficult/expensive. It's like locking your car so the stereo thief breaks into the unlocked car instead. It won't block a highly motivated/funded attacker, but will keep out some script kiddies. If the cost of splitting the key is near zero, it might be a good benefit/cost tradeoff.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:14 UTC (Thu) by beoba (guest, #16942) [Link] "At these temperatures (around -50 °C) you can remove the chips from the computer and let them sit on the table for ten minutes or more, without appreciable loss of data." So you can probably just pull the RAM modules and stick em into a machine that reads their contents.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:32 UTC (Thu) by patrick_g (subscriber, #44470) [Link] >>The only idea that comes to mind is programming the system firmware to "wipe" all system RAM on startup, overwriting the contents of RAM with zeroes prior to loading any operating system. I'm not sure if that would help, though.If you click on the article link there is an answer to your idea : "An adversary can remove the chips from the target computer and transplant them into another computer that doesnt overwrite RAM on startup".
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:41 UTC (Thu) by flewellyn (subscriber, #5047) [Link] Yes, I know; I asked that question there. :-)
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:38 UTC (Thu) by rvfh (subscriber, #31018) [Link] I think what you really want to do is wipe at shut down, and I am sure this is dead easy to program into the Linux kernel ! Don't know how fast that is, but start-up memory test being a few seconds, I'd guess the same time or less to wipe a few GB of memory.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 16:46 UTC (Thu) by Los__D (guest, #15263) [Link] And the attacker just pulling the plug, when the s/he obviously has access to the machine is avoid how?
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 17:35 UTC (Thu) by JamesErik (subscriber, #17417) [Link] Surely there's enough capacitance in modern motherboards to provide a few milliseconds of CPU time when power is lost such that the kernel could detect the start of a power loss event and can tell the crypto-disk driver to erase its encryption keys? Or not?
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 17:55 UTC (Thu) by cventers (subscriber, #31465) [Link] Perhaps, but I suspect that even that would not be enough. Imagine an FBI raid... they bring along their PC, plug it in and get it ready, open your case, blast off your RAM with the duster, and then perhaps pull the DIMMS out of your running system and slap them into theirs. No power loss event, and no interrupt telling the kernel "Quick! Someone is unplugging the DIMM, write zeros to it before they can finish!" I'm glad this research was done, but reading something like this is unsettling for encryption in general. I think what we need is some kind of 'secure DIMM' that uses capacitance and sets all its bits to 0 or 1 the moment it loses power. Or perhaps it could somehow drain all rows and columns to a big load? (Disclaimer: I'm not an EE)
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 17:59 UTC (Thu) by tcoppi (guest, #44423) [Link] I think using superglue/epoxy on the chips combined with power-on memory checking and disabling of external boot devices(such as network or usb booting) should be able to mitigate this attack well enough, at least for all but the most paranoid of us.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 18:43 UTC (Thu) by zlynx (subscriber, #2285) [Link] Use a case-open sensor or five to trigger thermite "heat sinks" (heh) mounted on your hard disks and RAM. Physical destruction is the only way to be sure.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 19:28 UTC (Thu) by cventers (subscriber, #31465) [Link] As long as we're playing that game, what if they splice open the power cable and attach their own source so that they can untether it from your outlet, then take it back to their lab where they can use ultrasound to find the case sensors, then cut around them? :p Seriously though, there is often a cost factor in making security decisions, and I would like to think that so-called "secure DIMMs" that are designed to quickly empty themselves upon power loss become prevalent. Extraction could still be possible by carefully applying the right power to the right pins while removing the chips, but at least it wouldn't be as easy as the attack described in this paper appears to be.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 22:36 UTC (Thu) by endecotp (guest, #36428) [Link] > what if they splice open the power cable and attach their > own source so that they can untether it from your outlet, > then take it back to their lab I think you were joking, but that's actually exactly what they do: http://www.engadget.com/2007/11/06/wiebetech-hotplug-lets...
Cold Reboot Attacks on Disk Encryption Posted Feb 23, 2008 22:42 UTC (Sat) by macc (subscriber, #510) [Link] GPS? tilt sensor or one of these little gyro thingies used for model helicopters? Then moving the box would shut the OS down? G! MACC
Cold Reboot Attacks on Disk Encryption Posted Feb 26, 2008 5:52 UTC (Tue) by njs (guest, #40338) [Link] Naw, don't shut down. Presumably the real solution is to every n minutes throw away crypto keys and at the next attempt to access the encrypted data, require the passphrase be re-entered. (We assume that attackers can't or won't breach the case to suck things out in less than n minutes.) Then if you want to get fancy, you can also add accelerometers to the list of things that trigger the crypto keys being flushed. Nagging the user on a timer is somewhat annoying, of course, but should be practical if one really has secret data (and if you segregate out the actually secret stuff from the everyday stuff). It might even be a feature to bug the user occasionally while they have secret data open, to remind them of this fact -- sort of a modern equivalent to root's "#" prompt.
Cold Reboot Attacks on Disk Encryption Posted Feb 25, 2008 13:11 UTC (Mon) by forthy (guest, #1525) [Link] "Intrusion detection" is the key here. Add an acceleration sensor to your case, and make sure to set the thresholds so that any small case movement causes the system to erase its memory and shut down. Optimally screw the case to the wall so that you have to open it to move it, anyway, and no accidental kick with your foot will trigger the sensor. And remember to put trip wires around your door - as typical geek, you are supposed to be lonely (you know all your friends only by their IRC nicks, and all women you know have the jpeg surname ;-), so the likelyhood that someone else trips over your trip wires is extremely small ;-).
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 17:58 UTC (Thu) by alecs1 (guest, #46699) [Link] Interesting question. Some folks told me once that power failed gradualy (a few until seconds until incandescent bulbs went out completely) and they saw the Windows shutdown screen, so probably the OS tried to go down in an emercency. I never knew if to believe them or not.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 17:55 UTC (Thu) by JoeBuck (subscriber, #2330) [Link] It's not clear that this attack provides much additional capability to either industrial spies or cops.This attack relies on getting a computer that is still running, so that you can either chill the RAM or immediately transfer the data out. But there are other attacks available in that case. If it's a laptop, you can just take the machine, leaving the power running. Even if not, there is technology available to keep the machine running (saw that on Schneier's blog. If someone has physical access to your machine when it's powered on and the encrypted disk is mounted, they have your data. As a practical matter, the key can be erased from RAM by an appropriate overwriting sequence (like those used for memory tests) that should suffice to prevent recovery. This could happen on shutdown, and the user could also be given a "panic button".
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 18:52 UTC (Thu) by alecs1 (guest, #46699) [Link] Two comments about available techonogy(I don't know how usefull): 1.At least in Romania and Germany they have Schuko (http://en.wikipedia.org/wiki/Schuko) as a standard, the pins are 19 mm and the hole in the socket almost as deep as the lenght of the pins. Someone would have to chop the plug seriously in order to get to the pins, I tryed right now, and there is no space to do anything without choping the plug/socket. (these schuko will get replaced, you need such force to pull them out that many times you also pull the socket from the wall). 2.Does this represent such an inovation that it will get a patent? I remember powering a socket like that myself, out of curiosity, this is a trivial idea.
Plug too hard? Jack the cable. Posted Feb 22, 2008 3:02 UTC (Fri) by midg3t (subscriber, #30998) [Link] Just remove the plastic sheath from the cable and poke your power source into the live & neutral wires. Unplug machine, profit.
Cold Reboot Attacks on Disk Encryption Posted Feb 22, 2008 4:20 UTC (Fri) by knobunc (subscriber, #4678) [Link] http://www.schneier.com/blog/archives/2008/02/hotplug_1.html has some of the relevant tech they use for this. I looked at the site and they have a few suggested methods for hooking in to the target computer's power: The Easy: If it is plugged into a power strip, you plug their UPS into the same strip then pull the plug to mains power. Take the computer, power strip and UPS off to your lab. (Same works if the outlet has 2 sockets, just unplug from the one that the computer is not using and plug the ups into that then take the outlet with you) The Harder: If there is no power strip, remove the outlet from the wall (while the computer is still plugged in). Attach some special jumper cables to the screws on the back and cut (physically) the mains power. The Hardest: Strip the power cord to the computer and attach the UPS wires directly to the wires inside the cord. -ben
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 18:44 UTC (Thu) by freemars (subscriber, #4235) [Link] A possible work around -- an expensive one -- could be to store part of the key in a CPU register (perhaps one of those 128-bit SSE registers). Not only would this require kernel completely rewritten to leave that register alone, it would probably slow the operating system by something like 1/(#_of_registers_in_this_CPU). Having to guess the final 128 bits of the key would at least annoy brute-force attackers.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 19:21 UTC (Thu) by ncm (subscriber, #165) [Link] Or, alternatively, in some other device: perhaps the graphics chip. On any given system there's some bit of complicated hardware that is not used. On many systems the 3D features are never used (e.g. Nvidia chip w/ nv driver). Others have firewire, or an MMC slot. For security, it could be an advantage that the key is stored at different addresses on different machines, particularly if random values are stored in the others, and the actual location used on any given boot is chosen from among them at random.
Cold Reboot Attacks on Disk Encryption Posted Feb 21, 2008 20:10 UTC (Thu) by elicriffield (guest, #33738) [Link] If your seizing a computer that is on, why would you ever shut it off Just use this device to move a running computer without it ever going to sleep or lose power, even if its plugged directly into the wall. http://www.youtube.com/watch?v=erq4TO_a3z8 Disk Encryption only works when the computer is off, and now we know it only works when the computer has been off for a min or two. Hole disk Encryption is still very effective for transporting laptops around, the most common cause of data lose. Anything you have thats important should be in an encrypted file thats not decrypted when your not using it, then even if they get to your hard drive your important data wont be compromised. You should be more worried about the TSA forcing you to give you there password: http://www.schneier.com/blog/archives/2008/02/us_customs_... Eli Criffield
Cold Reboot Attacks on Disk Encryption Posted Feb 22, 2008 16:04 UTC (Fri) by JoeBuck (subscriber, #2330) [Link] The "min or two" could be eliminated by having the shutdown or hibernate sequence wipe the key.
Cold Reboot Attacks on Disk Encryption Posted Feb 22, 2008 8:16 UTC (Fri) by xorbe (subscriber, #3165) [Link] epoxy them in! ;-)
Cold Reboot Attacks on Disk Encryption Posted Feb 22, 2008 17:35 UTC (Fri) by freehat (guest, #45283) [Link] This is an OLD article - but this should give you some more reasons to feel insecure ;) http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html
Cold Reboot Attacks on Disk Encryption Posted Feb 23, 2008 0:52 UTC (Sat) by jcm (subscriber, #18262) [Link] The crux of the matter is that people are lazy. They don't want to reenter their password on resume, or after leaving the machine unattended, so passwords are retained in memory unnecessarily. As a partial aide, perhaps we should store passwords physically in the first few pages of RAM after any machine vectors - that would make it more likely you would have to remove the chips to read the important bits. Jon.
Isn't this exactly why the TCPA chip was invented ? Posted Feb 23, 2008 9:21 UTC (Sat) by ernest (subscriber, #2355) [Link] From the story the TCPA will not give away it's keys. I hate to say this, but I have less objections to that chip on my motherboard now. Ernest.
Isn't this exactly why the TCPA chip was invented ? Posted Feb 24, 2008 16:56 UTC (Sun) by ballombe (subscriber, #9523) [Link] > From the story the TCPA will not give away it's keys. Really ? I rather read it as saying that TCPA does not protect against this attack.
The TCPA does not address this scenario at all Posted Feb 26, 2008 17:28 UTC (Tue) by hmh (subscriber, #3838) [Link] While the TPCA doesn't give away its storage encryption key easily, you still need to have the unwrapped (aka unprotected) session key for your encrypted disk somewhere in main memory to work with it. The complete fix for this problem really goes through extra hardware. Designing extra-volatile memory is easy, but you need to get that memory inside a more protected location (like inside the MCH, CPU, or data storage unit itself). And an attacker with enough resources would still be able to get to it. What we can do easily, is to reduce the windows of opportunity where the keys are available unprotected in RAM, which is good enough for a lot of scenarios. Frankly, if it can be made safe enough that regular laptop and data thieves can't get to the data, I would be personally happy enough.
Cold Reboot Attacks on Disk Encryption Posted Feb 28, 2008 7:25 UTC (Thu) by jimparis (subscriber, #38647) [Link] The 99.9% solution that I'm happy with is: never leave my computer unattended while the hard drive encryption keys are present in RAM. That means I never lock the screen and walk away, and I never suspend to ram and walk away. I just suspend to disk and power off completely. The only new thing this research pointed out to me is just how long it takes for unpowered room-temperature RAM to lose its contents -- I suspected it was within a second but it's a bit longer than that. Anyway, this small window can be fixed by modifying the suspend-to-disk process to also clear the RAM right before poweroff (I'm still in control of the laptop at this point, so nobody can interrupt it).
|
|||||||||||||