User`s guide

220 Copyright © Acronis International GmbH, 2002-2012
8.8.4.1 Functioning principles of Information wiping methods
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's and 0's (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.
Methods offered by national standards provide (single or triple) recording of random symbols to disk
sectors that are straightforward and arbitrary decisions, in general, but still acceptable in simple
situations. The most effective information-wiping method is based on deep analysis of subtle
features of recording data to all types of hard disks. This knowledge speaks of the necessity of
complex multipass methods to guarantee information wiping.
The detailed theory of guaranteed information wiping is described in an article by Peter Gutmann.
Please see:
Secure Deletion of Data from Magnetic and Solid-State Memory at
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html.
8.8.4.2 Information wiping methods used by Acronis
The table below briefly describes information wiping methods used by Acronis. Each description
features the number of hard disk sector passes along with the number(s) written to each sector byte.
The description of built-in information wiping methods
No.
Algorithm (writing method)
Passes
Record
1.
United States Department of
Defense 5220.22-M
4
1
st
pass – randomly selected symbols to each byte of
each sector, 2 – complementary to written during the
1
st
pass; 3 – random symbols again; 4 – writing
verification.
2.
United States: NAVSO
P-5239-26 (RLL)
4
1
st
pass – 0x01 to all sectors, 2 – 0x27FFFFFF, 3 –
random symbol sequences, 4 – verification.
3.
United States: NAVSO
P-5239-26 (MFM)
4
1
st
pass – 0x01 to all sectors, 2 – 0x7FFFFFFF, 3 –
random symbol sequences, 4 – verification.
4.
German: VSITR
7
1
st
– 6
th
– alternate sequences of: 0x00 and 0xFF; 7
th
– 0xAA; i.e. 0x00, 0xFF, 0x00, 0xFF, 0x00, 0xFF, 0xAA.
5.
Russian: GOST P50739-95
1
Logical zeros (0x00 numbers) to each byte of each
sector for 6
th
to 4
th
security level systems.
Randomly selected symbols (numbers) to each byte
of each sector for 3
rd
to 1
st
security level systems.
6.
Peter Gutmann's method
35
Peter Gutmann's method is very sophisticated. It's
based on his theory of hard disk information wiping
(see Secure Deletion of Data from Magnetic and
Solid-State Memory).
7.
Bruce Schneier's method
7
Bruce Schneier offers a seven-pass overwriting
method in his Applied Cryptography book. 1
st
pass –