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Full disk encryption isn’t quite dead

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.

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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.

Manipulating cold memory chips isn't for the faint at heart. Over the past two years, other researchers realized they could capture memory on powered-up computers by using the 1394 FireWire port found on most higher-end laptops. Here's one discussion detailing how to crack BitLocker using the FireWire memory attack [PDF].

Exploitation via FireWire:Prolific crypto- and password-cracking vendor Passware recently announced that it could crack both BitLocker- and TrueCrypt-protected disk volumes using the FireWire method. Theoretically, one can carry off similar attacks via a DMA-enabled port, such as PCI. These attacks can ultimately be successful against any software crypto product that does not use specialized hardware.

Dmitry Sumin, president of Passware, confirmed the news: "I think we could [crack] any of the popular [software FDE products]. It's a question of time and developing the key-finding algorithm."

The defense I discussed earlier against cold boot attacks can be used to defeat FireWire attacks. You can also beat them by simply disabling your FireWire port until needed. I asked Sumin if any of Passware's FDE-cracking products could break into a non-powered-on computer or if the 1394 port was disabled. He said, "No. We have a brute-force attack, but the encryption is rather secure if you have a good enough password."

The FireWire port can often be enabled or disabled through a computer's BIOS settings or in Device Manager within Microsoft Windows. Most Linux and Unix flavors allow the FireWire port to be disabled using a boot-up command switch, a recompiled kernel, or a command-line instruction — although the last option usually does not survive reboots. Most Mac support blogs suggest unloading the AppleFWOHCI.kext kernel extension.

Unfortunately, if you have PCI, PCMCIA, or related card slots, someone can slip in a FireWire adapter card, and it will function as well as an enabled FireWire port. I've seen this attack mode demonstrated, and I was impressed. Turning off all DMA-enabled ports is harder to do and will probably significantly affect performance (and possibly operating system stability), so I'm not sure I recommend it. Still, you can find coverage of a wide range of port-memory attacks and mitigations across multiple platforms. If you can't keep your powered-on computers physically secure when not in use, keep them powered off or in hibernation mode.

One way to prevent memory attacks is not to store the decryption key in normal memory. The Trusted Platform Module (TPM) chip from the Trusted Computing Group is an attempt to provide greater protection to crypto keys using specialized hardware. Most enterprise-class computers now come with a TPM chip, and several crypto vendors can take advantage of it, including Microsoft with BitLocker.

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