Appendix A. Hard Disk
Wiping Algorithms
Information removed from a hard disk drive by non-secure means (for
example, by simple Windows delete) can easily be recovered. Utilizing
specialized equipment, one may also be able to recover even repeatedly
overwritten information. Therefore the problem of guaranteed data wiping is
vital as never before.
The guaranteed wiping of information from magnetic media (e.g. a hard disk
drive) means the impossibility of data recovery by a qualified specialist with
the help of any known tools or recovery methods.
This problem can be explained in the following way: Data is stored on a hard
disk as a binary sequence of 1 and 0 (ones and zeros), represented by
differently magnetized parts of a magnetic disk.
Generally speaking, a 1 written to a hard disk is read as 1 by its controller,
and 0 is read as 0. However, of you write 1 over 0, the result is conditionally
0.95 and vice versa – if 1 is written over 1 the result is 1.05. These
differences are irrelevant for the controller. However using special
equipment, one can easily read the «underlying» sequence of 1 and 0.
It only requires specialized software and inexpensive hardware to read data
«deleted» this way by analyzing magnetization of hard disk sectors, residual
magnetization of tracksides and/or by using current magnetic microscopes.
Writing to magnetic media leads to subtle effects summarized as follows:
every track of a magnetic disk stores an image of every record ever written to
it, but the effect of such record (magnetic layer) becomes more subtle as
time passes.
A.1 Information Wiping Algorithms Functioning Principles
Physically the complete wiping of information from a hard disk involves the
switching of every elementary magnetic area of the recording material as
many times as possible by writing specially selected sequences of logical 1
and 0 (also known as samples).
Using logical data encoding methods in current hard disks, you can select
samples of symbol (or elementary data bit) sequences to be written to
sectors in order to repeatedly and effectively wipe confidential information.
Algorithms offered by national standards provide (single or triple) recording of
random symbols to disk sectors that are straightforward and arbitrary
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