Gutmann method

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The Gutmann method is an algorithm for securely erasing the contents of computer hard drives, such as files. Devised by Peter Gutmann and Colin Plumb, it does so by writing a series of 35 patterns over the region to be erased.

The selection of patterns assumes that the user doesn't know the encoding mechanism used by the drive, and so includes patterns designed specifically for three different types of drives. A user who knows which type of encoding the drive uses can choose only those patterns intended for their drive. A drive with a different encoding mechanism would need different patterns. Most of the patterns in the Gutmann method were designed for older MFM/RLL encoded disks. Relatively modern drives no longer use the older encoding techniques, making many of the patterns specified by Gutmann superfluous.[1]

Contents

[edit] Technical overview

One standard way to recover data that has been overwritten on a hard drive is to capture the analog signal which is read by the drive head prior to being decoded. This analog signal will be close to an ideal digital signal, but the differences are what is important. By calculating the ideal digital signal and then subtracting it from the actual analog signal it is possible to ignore that last information written, amplify the remaining signal and see what was written before.

For example:

Analog signal:        +11.1  -8.9  +9.1 -11.1 +10.9  -9.1
Ideal Digital signal: +10.0 -10.0 +10.0 -10.0 +10.0 -10.0 
Difference:            +1.1  +1.1  -0.9  -1.1  +0.9  +0.9
Previous signal:      +11    +11   -9   -11    +9    +9

This can then be done again to see the previous data written:

Recovered signal:     +11    +11   -9   -11    +9    +9
Ideal Digital signal: +10.0 +10.0 -10.0 -10.0 +10.0 +10.0 
Difference:            +1    +1    +1    -1    -1    -1
Previous signal:      +10   +10   -10   -10   -10   -10

However, even when overwriting the disk repeatedly with random data it's theoretically possible to recover the previous signal. The permittivity of a medium changes with the frequency of the magnetic field[citation needed]. This means that a lower frequency field will penetrate deeper into the magnetic material on the drive than a high frequency one[citation needed]. So a low frequency signal will, in theory still be detectable even after it has been overwritten hundreds of times by a high frequency signal.

The patterns used are designed to apply alternating magnetic fields of various frequencies and various phases to the drive surface and thereby approximate degaussing the material below the surface of the drive[citation needed].

[edit] Method

An overwrite session consists of a lead-in of four random write patterns, followed by patterns 5-31, executed in a random order, and a lead-out of four more random patterns.

Each of patterns 5-31 was designed with a specific magnetic media encoding scheme in mind, which each pattern targets. The drive is written to for all the passes even though the table below only shows the bit patterns for the passes that are specifically targeted at each encoding scheme. The end result should obscure any data on the drive so that only the most advanced physical scanning (e.g. using a magnetic force microscope) of the drive is likely to be able to recover any data.

The series of patterns is as follows:

Gutmann overwrite method
Pass Data Written Pattern written to disk for targeted encoding scheme
In Binary notation In Hex notation (1,7) RLL (2,7) RLL MFM
1 (Random) (Random)
2 (Random) (Random)
3 (Random) (Random)
4 (Random) (Random)
5 01010101 01010101 01010101 55 55 55 100... 000 1000...
6 10101010 10101010 10101010 AA AA AA 00 100... 0 1000...
7 10010010 01001001 00100100 92 49 24 00 100000... 0 100...
8 01001001 00100100 10010010 49 24 92 0000 100000... 100 100...
9 00100100 10010010 01001001 24 92 49 100000... 00 100...
10 00000000 00000000 00000000 00 00 00 101000... 1000...
11 00010001 00010001 00010001 11 11 11 0 100000...
12 00100010 00100010 00100010 22 22 22 00000 100000...
13 00110011 00110011 00110011 33 33 33 10... 1000000...
14 01000100 01000100 01000100 44 44 44 000 100000...
15 01010101 01010101 01010101 55 55 55 100... 000 1000...
16 01100110 01100110 01100110 66 66 66 0000 100000... 000000 10000000...
17 01110111 01110111 01110111 77 77 77 100010...
18 10001000 10001000 10001000 88 88 88 00 100000...
19 10011001 10011001 10011001 99 99 99 0 100000... 00 10000000...
20 10101010 10101010 10101010 AA AA AA 00 100... 0 1000...
21 10111011 10111011 10111011 BB BB BB 00 101000...
22 11001100 11001100 11001100 CC CC CC 0 10... 0000 10000000...
23 11011101 11011101 11011101 DD DD DD 0 101000...
24 11101110 11101110 11101110 EE EE EE 0 100010...
25 11111111 11111111 11111111 FF FF FF 0 100... 000 100000...
26 10010010 01001001 00100100 92 49 24 00 100000... 0 100...
27 01001001 00100100 10010010 49 24 92 0000 100000... 100 100...
28 00100100 10010010 01001001 24 92 49 100000... 00 100...
29 01101101 10110110 11011011 6D B6 DB 0 100...
30 10110110 11011011 01101101 B6 DB 6D 100...
31 11011011 01101101 10110110 DB 6D B6 00 100...
32 (Random) (Random)
33 (Random) (Random)
34 (Random) (Random)
35 (Random) (Random)

Encoded bits shown in bold are what should be present in the ideal pattern, although due to the encoding the complementary bit is actually present at the start of the track.

[edit] Criticism

Some have criticized Gutmann's claim that intelligence agencies are likely to be able to read overwritten data.[2]

The delete function in most operating systems simply marks the space occupied by the file as reusable (removes the pointer to the file) without immediately removing any of its contents. At this point the file can be fairly easily recovered by numerous recovery applications. However, once the space is overwritten with other data, there is no known way to recover it. It cannot be done with software alone since the storage device only returns its current contents via its normal interface. Gutmann claims that intelligence agencies have sophisticated tools, among these magnetic force microscopes, that, together with image analysis, can detect the previous values of bits on the affected area of the media (for example hard disk).

This has not been proven one way or the other, and there is no published evidence as to intelligence agencies' current ability to recover files whose sectors have been overwritten, although published Government security procedures clearly consider an overwritten disk to still be sensitive.[3]

Companies specializing in recovery from damaged media cannot recover completely overwritten files. These companies specialize in the recovery of information from media that has been damaged by fire, water or otherwise. No private data recovery company claims that it can reconstruct completely overwritten data as of now.

Gutmann himself has responded to some of these criticisms and also criticized how his algorithm has been abused in an epilogue to his original paper, in which he states [1]:

“ In the time since this paper was published, some people have treated the 35-pass overwrite technique described in it more as a kind of voodoo incantation to banish evil spirits than the result of a technical analysis of drive encoding techniques. As a result, they advocate applying the voodoo to PRML and EPRML drives even though it will have no more effect than a simple scrubbing with random data. In fact performing the full 35-pass overwrite is pointless for any drive since it targets a blend of scenarios involving all types of (normally-used) encoding technology, which covers everything back to 30+-year-old MFM methods (if you don't understand that statement, re-read the paper). If you're using a drive which uses encoding technology X, you only need to perform the passes specific to X, and you never need to perform all 35 passes. For any modern PRML/EPRML drive, a few passes of random scrubbing is the best you can do. As the paper says, "A good scrubbing with random data will do about as well as can be expected". This was true in 1996, and is still true now. ”

[edit] Software implementations

[edit] See also

[edit] External links

[edit] Notes

  1. ^ a b Gutmann, Peter. (July 22-25, 1996) Secure Deletion of Data from Magnetic and Solid-State Memory. University of Auckland Department of Computer Science. Epilogue section. (writing, "In fact performing the full 35-pass overwrite is pointless for any drive since it targets a blend of scenarios involving all types of (normally-used) encoding technology, which covers everything back to 30+-year-old MFM methods (if you don't understand that statement, re-read the paper). If you're using a drive which uses encoding technology X, you only need to perform the passes specific to X, and you never need to perform all 35 passes. For any modern PRML/EPRML drive, a few passes of random scrubbing is the best you can do. As the paper says, "A good scrubbing with random data will do about as well as can be expected". This was true in 1996, and is still true now.").
  2. ^ "Can Intelligence Agencies Read Overwritten Data? A response to Gutmann.". http://web.archive.org/web/20080202160840/http://www.nber.org/sys-admin/overwritten-data-gutmann.html. 
  3. ^ "Clearing and Declassifying Electronic Data Storage Devices". http://www.cse-cst.gc.ca/documents/publications/gov-pubs/itsg/itsg06.pdf. 
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