"Sharon Nagao" <[EMAIL PROTECTED]> wrote: > . . . an application that can overwrite a file such that it is virtually > impossible to read the data in the file back from disk using forensic > methods
You need to thoroughly read Peter Gutmann's paper "Secure Deletion of Data from Magnetic and Solid-State Memory" [The Sixth USENIX Security Symposium Proceedings, 1996, pp. 77-89]: http://www.usenix.org/publications/library/proceedings/sec96/gutmann.html Briefly, the paper points out the following problems: 1) Magnetic force microscopy and scanning tunneling microscopy can recover pretty much any data that's *ever* been written to a disk! 2) Older data is harder to recover using MFM or STM, but not impossible. 3) Over-writing data with the correct set of bits depends in part on the recording method used by the disk itself. 4) The most effective solution is to physically destroy the media. So, without knowing *all* of the transformations that may happen to your data bits from the time they "leave" your program to the time they actually come to rest on the disk, it's quite difficult (if not impossible) to over-write your data so that it can't be recovered. Gutmann's paper discusses in details why "total erasure" is hard to achieve -- in part because it depends on the actual hardware you're trying to use. Another aspect discussed only in part by Gutmann is making sure the correct blocks are erased. You not only have to worry about data left in freed blocks, you also need to be concerned with bad block remapping. Even if you think you're wiping the whole disk surface, sensitive blocks may still remain. Again, if it's critical, smash the disk into very small pieces. Degaussing? Nope -- Gutmann shows how the energy levels needed for some (older?) disks is much higher than most degaussing units can produce. Home Depot usually has a very nice selection of sledge hammers -- a fiberglass-handled ten-pounder ought to do the job. :-) AdamM --- Send mail for the `bblisa' mailing list to `[EMAIL PROTECTED]'. Mail administrative requests to `[EMAIL PROTECTED]'.
