Contrary to what most people think, computer memory is not instantly erased when power is cut. Rather, it fades gradually over a period of seconds to minutes as charge leaks out of the DRAM cells.

We loaded a bitmap image into memory on a test computer, then cut power for varying intervals. After 5 seconds (left), the image is nearly indistinguishable from the original; it gradually becomes more degraded, as shown after the computer has been off for 30 seconds, 60 seconds, and 5 minutes. Even after this longest trial, traces of the original remain.

The decay shows prominent patterns. Some areas of the memory chip are wired to interpret lack of charge as a 0 bit, others as a 1 bit -- this results in the alternating horizontal bars. The fainter vertical bands are caused by physical variations in the chip, which cause charge to leak out slightly faster or slower in different areas.

Our research shows that this little-known phenomenon, called memory remanence, has dangerous security implications. For example, it could be used to break the disk encryption on a stolen laptop and reveal sensitive data.