Encrypting your laptop's hard drive does not keep your data safe, according to researchers at Princeton University.

They've discovered a way to steal the hard drive encryption key used by products such as Windows Vista's BitLocker or Apple's FileVault. With that key, hackers could get access to all of the data stored on an encrypted hard drive.

That's because of a physical property of the computer's memory chips. Data in these DRAM processors disappears when the computer is turned off, but it turns out that this doesn't happen right away, according to Alex Halderman, a Princeton graduate student who worked on the paper.

In fact, it can take minutes before that data disappears, giving hackers a way to sniff out encryption keys.

For the attack to work, the computer would have to first be running or in standby mode. It wouldn't work against a computer that had been shut off for a few minutes because the data in DRAM would have disappeared by then.

The attacker simply turns the computer off for a second or two and then reboots the system from a portable hard disk, which includes software that can examine the contents of the memory chips. This gives an attacker a way around the operating system protection that keeps the encryption keys hidden in memory.

"This enables a whole new class of attacks against security products like disk encryption systems that have depended on the operating system to protect their private keys," Halderman said. "An attacker could steal someone's laptop where they were using disk encryption and reboot the machine ... and then capture what was in memory before the power was cut."

Some computers wipe the memory when they boot up, but even these systems can be vulnerable, Halderman said. Researchers found that if they cooled down the memory chips by spraying canned air on them, they could slow down the rate at which memory disappeared. Cooling chips down to about -50 degrees Celsius gave researchers time to power down the computer and then install the memory in another PC that would boot without wiping out the data. "By cooling the chips we were able to recover data perfectly after 10 minutes or more," Halderman said.