tcmay@got.net (Timothy C. May) writes:
Degaussing the media (running a household magnet over it :-) may be an optio
Ordinary household magnets fail for a couple of reasons:
I've just established experimentally that thoroughly running a household magnet over a 3.5" floppy messed up less than 1/2 the sectors I tried to read. Not a good option even for floppies. (Actually, there _was a smiley up there)
1. Does anyone know a cheap way to recover the traces of the previous (overwritten) recordings on the media?
There are custom drives for various media which have multiple heads, and heads that can be "jogged" a little bit. This allows, I have read, the subtle variations of multiple writes to be extracted.
Much more expensive would be various electron microscope-based imaging methods to directly image the domains and extract subtle signs of past write cycles.
I'll go on a tangent (this has more of a stego than crypto code relevance): In the early '80s there was much activity related to floppy disk based copy protection schemes (we got our first PC in Dec 81; most folks today know dongles, but may not remember disk-based copy protection). The original IBM PC came with 360K 5.25" floppy drives and a very smart floppy disk controller chip that was capable of much more than what the IBM BIOS normally asked of it; and even the BIOS was capable of much more (floppy disk related) than PC DOS required. One of the neater tricks I've seen were the so called "weak bits". One could confuse the FDC and write a sector in such a way that when subsequently someone read it, he saw 1's some of the times and 0's at other times. Naturally, the FDC noted the CRC error on the sector. The copy protection checker could read the sector several times into different buffers and see that it got different results every time. I rummaged around my archives and found an assembly program (about 10K) that I once wrote (dated Jan 84) which I think did exactly this. I can e-mail it to anyone who cares to take a look. (Disclaimer: I no longer remember what it does, but I think this is the one with weak bits.) I would not be very surprised if it turned out to be possible to confuse the floppy disk controller (or some hard disk controllers) by software alone, so that instead of operating "correctly" and reading the most recently written data, it would operate "incorrectly" and pick up traces of the overwritten bits from the media. Jim Bell mentioned the trick of hiding information into 'extra' tracks and sectors not used by the usual DOS formatting. It's very old too. I think I saw copy protection schemes circa 1982 that hid important data on tracks 41--43. 360K diskettes normally had 40 tracks. If the diskette was copies by DISKCOPY, it didn't know about the extra tracks, and the copy didn't have the info (usually, a piece of the program). It's very easy to do with just BIOS calls to format/read/write the track. Problem is, many cheap floppy drives these days aren't capable of seeking beyond track 80 when the FDC asks them to. You can write the data there and give the floppy to a friend who won't be able to read it from there. Microsoft uses a variation of this scheme when it formats its distribution diskettes for some products with additional sectors on every track (and presumably a smaller inter-sector gap, and good media). Some may recall that the original PC DOS 1.x formatted disks with 8 sectors/track (for 160K/320K) and 2.x and later started formatting 9 sectors. There was a popular hack to put 10 sectors on a track (including a DOS device driver to read such disks). This too can be accomplished by BIOS without any FDC hacking. (Thanks also to tallpaul for info on Vogons) --- Dr. Dimitri Vulis Brighton Beach Boardwalk BBS, Forest Hills, N.Y.: +1-718-261-2013, 14.4Kbps