DoD 5220.22-M的說明

Use this seven-pass method for tighter security. Different patterns of

bytes are written to the disk as described in the table below. Using

this method is probably even safer than using the simple method (with 6

passes).

This method is described in the National Industrial Security Program

Operating Manual (NISPOM a.k.a. DoD 5220.22-M) of the US Department of

Defense (January 1995; chapter 8, section 3, 8-306. Maintenance).

The free disk space is overwritten seven times:

Pass Data

1 A random character, n = [0, 255]

2 A random character, n

3 Complement of previous character, ~n

4 A random character, n

5 A random character, n

6 Complement of previous character, ~n

7 A random character, n

However, if you want to be absolutely sure of your data security, use the Gutmann method, below

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Gutmann method的說明

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This method offers the most tight security for the whole range of disk

drive types. You should use this method if your data is very valuable

and you think your disk could possibly be scrutinized by expensive,

specialized equipment.

The method used by Clean Disk Security is based on that described in

the paper Secure Deletion of Data from Magnetic and Solid-State Memory,

by Peter Gutmann (Department of Computer Science University of Auckland.

pgut001@cs.auckland.ac.nz), and is included in Clean Disk Security with his permission.

The paper was first published in the Sixth USENIX Security Symposium Proceedings, San Jose, California, July 22-25, 1996.

The first four and last four passes are pseudo-random data (created

with additive congruential pseudo-random number generator) and other

passes are made in random order.

Overwrite Data

PassNo. Data Written Encoding Scheme Targeted

1 Random

2 Random

3 Random

4 Random

5 01010101 01010101 01010101 0×55 (1,7) RLL MFM

6 10101010 10101010 10101010 0xAA (1,7) RLL MFM

7 10010010 01001001 00100100 0×92 0×49 0×24 (2,7) RLL MFM

8 01001001 00100100 10010010 0×49 0×24 0×92 (2,7) RLL MFM

9 00100100 10010010 01001001 0×24 0×92 0×49 (2,7) RLL MFM

10 00000000 00000000 00000000 0×00 (1,7) RLL (2,7) RLL

11 00010001 00010001 00010001 0×11 (1,7) RLL

12 00100010 00100010 00100010 0×22 (1,7) RLL

13 00110011 00110011 00110011 0×33 (1,7) RLL (2,7) RLL

14 01000100 01000100 01000100 0×44 (1,7) RLL

15 01010101 01010101 01010101 0×55 (1,7) RLL MFM

16 01100110 01100110 01100110 0×66 (1,7) RLL (2,7) RLL

17 01110111 01110111 01110111 0×77 (1,7) RLL

18 10001000 10001000 10001000 0×88 (1,7) RLL

19 10011001 10011001 10011001 0×99 (1,7) RLL (2,7) RLL

20 10101010 10101010 10101010 0xAA (1,7) RLL MFM

21 10111011 10111011 10111011 0xBB (1,7) RLL

22 11001100 11001100 11001100 0xCC (1,7) RLL (2,7) RLL

23 11011101 11011101 11011101 0xDD (1,7) RLL

24 11101110 11101110 11101110 0xEE (1,7) RLL

25 11111111 11111111 11111111 0xFF (1,7) RLL (2,7) RLL

26 10010010 01001001 00100100 0×92 0×49 0×24 (2,7) RLL MFM

27 01001001 00100100 10010010 0×49 0×24 0×92 (2,7) RLL MFM

28 00100100 10010010 01001001 0×24 0×92 0×49 (2,7) RLL MFM

29 01101101 10110110 11011011 0x6D 0xB6 0xDB (2,7) RLL

30 10110110 11011011 01101101 0xB6 0xDB 0x6D (2,7) RLL

31 11011011 01101101 10110110 0xDB 0x6D 0xB6 (2,7) RLL

32 Random

33 Random

34 Random

35 Random