Protect your crypto key from the cold

LAS VEGAS'The report in February from the Princeton Center for IT Policy that data stored in Dynamic Random Access Memory (DRAM) chips is longer-lived than commonly believed had a chilling effect on the cryptography community.

Tools have been developed that can recover encryption keys from DRAM if you can get access to the computer quickly enough after it has been powered down. Data in DRAM can be made even more stable by cooling down the chips, giving a forensics analyst or a thief minutes or even hours to recover those keys, rather than seconds.

This technique of accessing slowly fading memory is called a cold boot attack.

'This contradicted the assumption that everyone has about what happens when you turn your computer off,' said Patrick McGregor, chief executive officer of BitArmor Systems Inc. of Pittsburgh. 'With a click of the power button the information [is supposed to] dissipate. But it doesn't. When it comes to encryption, this is a problem.'

But not to worry. BitArmor has developed a software suite to defend against cold boot attacks. McGregor, who has a doctorate from Princeton and has done cryptographic research with some of the authors of the cold boot report, demonstrated those tools yesterday at the Black Hat Briefings. They have not been formally announced, but they are being incorporated in the company's DataControl product, an endpoint data protection and management system.

The problem of persistent DRAM memory is not entirely new and the forensics community has been aware of it for some time, according to computer security experts at the National Institute of Standards and Technology. But general knowledge of it and the availability of tools that can recover keys make it a problem, McGregor said. 'This is a big deal.'

Disk encryption, which can be compromised by these attacks, is an increasingly important tool in protecting data from improper exposure. According to some estimates, as much as $200 million was spent on disk encryption tools in 2007.

'Most experts assume that a computer's memory is erased almost immediately when it loses power, or that whatever data remains is difficult to retrieve without specialized equipment,' the researchers write in their report, titled 'Lest We Remember: Cold Boot Attacks on Encryption Keys.' 'We show that these assumptions are incorrect. Ordinary DRAM chips typically lose their contents gradually over a period of seconds, even at standard operating temperatures and even if the chips are removed from the motherboard, and data will persist for minutes or even hours if the chips are kept at low temperatures.'

By chilling chips to prolong memory and using algorithms to recognize and recover cryptographic keys, researchers were able to defeat several disk encryption systems, including BitLocker, TrueCrypt and FileVault.

The researchers built their work on the known fact that system memory persists for a matter of seconds after power is removed from the DRAM. 'In most cases, we observed that almost all bits decayed at predictable times and to predictable 'ground states' rather than to random values,' they wrote. 'We also confirmed that decay rates vary dramatically with temperature.'

Discharging a compressed air keyboard duster directly onto the chip lowered the temperature to 50 degrees below zero Celsius (-58 degrees Fahrenheit). 'At these temperatures, we typically found that fewer than 1 percent of bits decayed even after 10 minutes without power. To test the limits of this effect, we submerged DRAM modules in liquid nitrogen (ca. -196 C) and saw decay of only 0.17 percent after 60 minutes out of the computer.'

Using algorithms, researchers were able to reconstruct 128-bit AES keys on which 10 percent of the bits had decayed in a matter of seconds. They also developed reconstruction techniques for DES and RSA keys.

Key recovery does not have to rely on super-chilled chips if a thief can get his hands on a computer that has merely been put in a sleep or hibernation mode. Because power remains on in sleep mode, data remains in the chips. A thief with software tools on a USB device can reboot the stolen computer, cutting power to the chips for just a blink, and then recover persistent data before it has a chance to decay.

With this type of attack, 'your chances of success are about 100 percent,' McGregor said.

To date, defense against cold boot attacks have focused on behavior: Turn your computer off when not in use rather than relying on the sleep mode, and keep your eyes on it for several minutes.

BitArmor has incorporated software in DataControl that defends against four avenues of attack. It can securely overwrite memory and purge keys from DRAM before the system is powered down; if booted to another operating system it can overwrite keys before rebooting; and it can create a secure enclave in Pentium chips to hold keys secure from a cold boot attack. For computers that have temperature sensors, originally intended to protect against overheating, it can use those sensors to detect chilling and respond by purging keys.

Although not yet announced, the software is included in the DataControl agent that resides on a protected client. The feature has to be turned on by the policy administrator but does not require any other policy configuration.

About the Author

William Jackson is a Maryland-based freelance writer.


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