Full disk encryption isn't quite dead

Basic security measures can thwart innovative attempts to crack hard-drive encryption

At least once a month, it seems some vendor or techie claims to have broken a version of a hard drive full-disk encryption (FDE) program scheme, whether it's from Microsoft (my full-time employer), BitLocker, open source favorite TrueCrypt, or some other variant. All the stories and the hype are enough to make one wonder if FDE is dead.

The brief -- and slightly qualified -- answer is no. There are a handful of clever attacks, as well as software to make them easier to pull off. Luckily there are easy ways to prevent most of them. We will start, however, with an attack that doesn't have an easy defense.

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Cold boot attack
In February 2008, a team including Princeton's Dr. Edward Felton -- one of the world's premier computer security researchers -- used an interesting intrinsic property of computer memory to successfully hack BitLocker [PDF]. It turns out that computer memory chips will hold their contents from a few seconds to a few minutes after the computer's power is turned off. Further, lowering the temperature or freezing the chips enables the contents to remain in play much longer -- enough time to be transferred to another specialized analysis computer so that the data can be copied to permanent storage. The attack team could then search for the primary BitLocker encryption key and unlock the data.

The "cold boot" attack is perhaps the toughest attack to defend against on a computer without specialized crypto-hardware. The flaw lies more with computer memory than the involved crypto. All software-based crypto has to eventually place the decryption key in normal memory in an unprotected state so that it can be used to decrypt the hard drive. An attacker can always find the unprotected key when he or she has a copy of memory to examine.

This plan requires the attacker to somehow acquire the victim's computer while it's powering down, just after it's powered down, or when it's coming back up from a suspended or standby state. Then the attacker has to freeze the chips, transfer them to another specialized computer, and use specially built software to find the key for the FDE cipher. If you're worried about this attack, make sure your unattended, powered-on computers have good physical security; alternatively, consider using hardware crypto solutions that are resistant to cold boot attacks.

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