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Not random

Posted by: Anonymous Coward on March 08, 2006 02:41 AM
Unless someone has gone and changed it, shred does not "overwrites a file 25 times with random data". Some of the passes are random, but most of them are designed to effectively degauss the disk. This is done by writing bit patterns, which when translated using 1,7 or 2,7 RLL (or a few other encoding schemes) write to the disk with long runs of one, then the other polarity. Subsequent passes shorten the length of the runs. The reason for this is that low frequencies (long runs) penetrate deeper into the disk surface and can't actually be completely erased by higher frequencies (short runs). Actually writing random data will still leak information.

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