No, more than once is not overkill. Super-duper computer forensics labs can "see through" single writes without too much problem. You've got to do it several times to really give these guys a real problem.

I just as intrigued, though, by the fact that it's really easy to read letters without opening envelopes, even correspondence which has be folder over serveral times and placed in a "secure" envelope.

No, my friend, you've got to overwrite many times to approach secure, and even then there are no guarrantees against the most motivited government agencies.

--Richard

I once saw a post on Usenet claiming that all the data of the entire federal government is kept on a single 20 MB hard disk. They keep overwriting the data, but the FBI/CIA/NSA can read down through the "layers", so no problem...

A silly joke, of course...

Seriously, reading overwritten data is possible because each little section of disk that's supposed to be a 0 or 1 actually has many magnetic grains in it. Writing a 0 or 1 will flip most, but never all, of the grains. FBI & co. can look at something that seems to be, say, all 0's, and detect residual 1's at certain spots---grains that didn't make the switch to 0's---, and from that, recover a lot of the original data.

I think this can be done with certain types of microscopes---atomic force microscope (AFM) maybe? I forget---if you happen to have one lying around... :-)

Anyway, if you overwrite seven times, it's supposed to be unlikely that any individual grain would stay unchanged seven times in a row.