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Cold boot attacks, used to extract sensitive data such as encryption keys and passwords from system memory, have been given new blood by researchers from F-Secure. First documented in 2008, cold boot attacks depend on the ability of RAM to remember values even across system reboots. In response, systems were modified to wipe their memory early during the boot process—but F-Secure found that, in many PCs, tampering with the firmware settings can force the memory wipe to be skipped, once again making the cold boot attacks possible.
The RAM in any commodity PC is more specifically called Dynamic RAM (DRAM). The “dynamic” here is in contrast to the other kind of RAM (used for caches in the processor), static RAM (SRAM). SRAM retains its stored values for as long as the chip is powered on; once the value is stored, it remains that way until a new value is stored or power is removed. It doesn’t change, hence “static.” Each bit of SRAM typically needs six or eight transistors; it’s very fast, but the high transistor count makes it bulky, which is why it’s only used for small caches.
DRAM, on the other hand, has a much smaller size per bit, using only a single transistor paired with a capacitor. These capacitors lose their stored charge over time; when they’re depleted, the DRAM no longer retains the value it was supposed to remember. To handle this, the DRAM is refreshed multiple times per second to top up the capacitors and rewrite the values being stored. This rewriting is what makes DRAM “dynamic.” It’s not just the power that needs to be maintained for DRAM; the refreshes also need to occur.