Description: 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.
This research is conducted by J. Alex Halderman, Seth D. Schoen, Nadia Heninger, William Clarkson, William Paul, Joseph A. Calandrino, Ariel J. Feldman, Jacob Appelbaum, and Edward W. Felten from Princeton university.
This video demonstrates the reasons behind this attack to be successful and then shows one illustrative attack scenario.DRM systems often rely on symmetric keys stored in memory, which may be recoverable using the techniques outlined in this video. As we have shown, SSL-enabled web servers are vulnerable, since they often keep in memory private keys needed to establish SSL sessions. Furthermore, methods similar to our key-finder would likely be effective for locating passwords, account numbers, or other sensitive data in memory.
There seems to be no easy remedy for these vulnerabilities. Simple software changes have benefits and drawbacks; hardware changes are possible but will require time and expense; and today’s Trusted Computing technologies cannot protect keys that are already in memory.
The risk seems highest for laptops, which are often taken out in public in states that are vulnerable to our attacks. These risks imply that disk encryption on laptops, while beneficial, does not guarantee protection. Ultimately, it might become necessary to treat DRAM as untrusted, and to avoid storing sensitive data there, but this will not be feasible until architectures are changed to give software a safe place to keep its keys.Important Links
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