Does RAM store data so that it can be retained after removal?
Is it possible for RAM to retain any data after power is removed? I don't mean within a few minutes such as cold boot Attacks but rather 24 hours plus.
Working with classified systems the policy always seems to treat RAM the same as disks and must be removed and disposed of according to classification. Is this a myth which has become standard practice or is there really a data security risk present?
This article analyses retention time for several DRAM chips. Among the relevant information to find the answer to does RAM store data, one may list the following: Retention time depends on a lot of things, including the values of neighbouring bits. A DRAM bit is a potential well, and it loses its contents by moving charges from or into neighbouring areas, so whether there is room in these neighbours matters. Temperature is very important for retention time (which is why cold-boot attacks insist on cold: if you plunge the machine in liquid nitrogen, you can keep the charges in place for substantially longer). At room temperature, typical retention time is counted in milliseconds, at best a few seconds, and, more importantly, the discharge is exponential in nature (it goes in e-Ct for some constant C), as could be expected (capacitors also work that way). So the remaining charge after 2 minutes will be half that after 1 minute; after 10 minutes you are down to a thousandth of the initial charge; after 20 minutes, a millionth; after 30 minutes, a billionth.
To sum up: 24 hours... forget it. You won't find meaningful data in DRAM that has been kept unpowered, at room temperature, after 24 hours (even if the room is, say, in Canada). This is for DRAM, where a stored bit can be envisioned as a charged capacitor. This is the kind of RAM commonly found in PCs for the last 20 years.
There also exists SRAM, where each bit is stored as the current state of a bistable circuit that consists of 6 transistors. SRAM is substantially faster than DRAM; it is also a lot more expensive. In PC, SRAM is used for cache (usually integrated in the CPU). Without power, SRAM loses any trace of its contents within microseconds.
There are some stories about bits being "burned" into RAM when the same value is stored for a long time in a specific emplacement in a chip. To the best of my knowledge, these stories are exactly that: stories. They come from "thought by analogy", by people who think of RAM in the same way as they think about CRT displays (which could have "burn-in" effects, hence the development of "screensavers"). I am not aware of any case where such stories were ever substantiated.But fears and doubts are powerful forces that cannot always be dispelled by the strongest logic.