According to researchers at Princeton University, it’s possible to recover encryption keys from memory for some time after a computer is powered down. Their paper, “Lest We Remember: Cold Boot Attacks on Encryption Keys,” begins with this abstract:
Contrary to popular assumption, DRAMs used in most modern computers retain their contents for seconds to minutes after power is lost, even at operating temperatures and even if removed from a motherboard. Although DRAMs become less reliable when they are not refreshed, they are not immediately erased, and their contents persist sufficiently for malicious (or forensic) acquisition of usable full-system memory images. We show that this phenomenon limits the ability of an operating system to protect cryptographic key material from an attacker with physical access. We use cold reboots to mount attacks on popular disk encryption systems — BitLocker, FileVault, dm-crypt, and TrueCrypt — using no special devices or materials. We experimentally characterize the extent and predictability of memory remanence and report that remanence times can be increased dramatically with simple techniques. We offer new algorithms for finding cryptographic keys in memory images and for correcting errors caused by bit decay. Though we discuss several strategies for partially mitigating these risks, we know of no simple remedy that would eliminate them
Check out the researchers’ video demo of the attack:
[kml_flashembed movie="http://www.youtube.com/v/JDaicPIgn9U" width="425" height="350" wmode="transparent" /]
While I don’t consider this a great concern for the average user, it’s a real problem in terms of corporate espionage and national security.
Aside from simply never using standby modes or screen locking, possible solutions would be for encryption programs to require two-factor authentication or for operating systems to securely erase memory as part of the shutdown routine. This article at SANS Internet Storm Center gives further insight into the issue.