Security Basics mailing list archives
Re: Wiping of Flash based Media.
From: MaddHatter <maddhatt+securitybasics () cat pdx edu>
Date: Wed, 23 Apr 2008 19:20:53 -0700
"Worrell, Brian" <BWorrell_isdh.IN.gov> said (on 2008/04/23):
The DoD hard drive wipe seems to be okay (not perfect I know.) for removing sensitive data from a hard drive, but do you think it is acceptable for an SSD or other Flash based storage? If a DoD wipe is not good, what are your thoughts on something that is, or would work?
Your approach will depend on your level of paranoia. As usual, the truly schizophrenic will only be happy with complete destruction (and not entirely without good reason). Modern high density Flash devices use wear-leveling. This means at any time the device could decide that block A (a random chunk of memory, probably several megabits large) is about to go belly-up, so it will move the data in block A to block Z (and remap A->Z). It won't erase block A, just sort of forget that it ever existed. You have no idea what potentially sensitive data was in block A before it got decommissioned. A sufficiently knowledgeable and determined attacker will be able to recover much of the data from block A, even after the device has otherwise ceased to function. An attacker might also be able to determine the relative stress seen by each cell. Erasing (usually, changing to a 11 state) is a stressful operation in Flash and causes measurable degradation. Cells that are frequently erased will appear different than cells that have been erased infrequently. Whether that's sensitive information depends on application and on details of the device operation (which you have to assume the attacker would know). If you're not quite that worried, overwriting once with zeroes, then ones, then zeroes is likely good enough. You could do ones, zeroes, ones -- what's relevant is that every single cell has been set to both its maximum and minimum state. Recovering any old data from the user-accessible (i.e. not decommissioned) blocks at that point is highly improbable. The variation in the state of the cells after such an operation is driven by process variation and intrinsic effects that swamp whatever historical state could plausibly remain.
Current thread:
- Wiping of Flash based Media. Worrell, Brian (Apr 23)
- Re: Wiping of Flash based Media. MaddHatter (Apr 24)
- Re: Wiping of Flash based Media. xgermx (Apr 24)
- Re: Wiping of Flash based Media. Ansgar -59cobalt- Wiechers (Apr 24)
- Re: Wiping of Flash based Media. xgermx (Apr 24)
- RE: Wiping of Flash based Media. Ramki B (Apr 28)
- Re: Wiping of Flash based Media. MaddHatter (Apr 24)