Gutmann method

The Gutmann method is an algorithm for securely erasing the contents of computer hard disk drives, such as files. Devised by Peter Gutmann and Colin Plumb and presented in the paper Secure Deletion of Data from Magnetic and Solid-State Memory in July 1996, it involved writing a series of 35 patterns over the region to be erased.

The selection of patterns assumes that the user does not know the encoding mechanism used by the drive, so it includes patterns designed specifically for three 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. Gutmann himself has noted that more modern drives no longer use these older encoding techniques, making parts of the method irrelevant. He said "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".[1][2]

Since about 2001, some ATA IDE and SATA hard drive manufacturer designs include support for the ATA Secure Erase standard, obviating the need to apply the Gutmann method when erasing an entire drive.[3] However, a 2011 research found that 4 out of 8 manufacturers did not implement ATA Secure Erase correctly.[4]

Technical overview

One standard way to recover data that have been overwritten on a hard drive is to capture and process the analog signal obtained from the drive's read/write head prior to this analog signal being digitized. This analog signal will be close to an ideal digital signal, but the differences will reveal important information. By calculating the ideal digital signal and then subtracting it from the actual analog signal, it is possible to amplify the signal remaining after subtraction and use it to determine what had previously been written on the disk.

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 is theoretically possible to recover the previous signal. The permittivity of a medium changes with the frequency of the magnetic field. This means that a lower frequency field will penetrate deeper into the magnetic material on the drive than a high frequency one. 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.

Method

An overwrite session consists of a lead-in of four random write patterns, followed by patterns 5 to 31 (see rows of table below), executed in a random order, and a lead-out of four more random patterns.

Each of patterns 5 to 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 0101010155 55 55100... 000 1000...
6 10101010 10101010 10101010AA AA AA00 100... 0 1000...
7 10010010 01001001 0010010092 49 24 00 100000...0 100...
8 01001001 00100100 1001001049 24 92 0000 100000...100 100...
9 00100100 10010010 0100100124 92 49 100000...00 100...
1000000000 00000000 0000000000 00 00101000...1000...
1100010001 00010001 0001000111 11 110 100000...
1200100010 00100010 0010001022 22 2200000 100000...
1300110011 00110011 0011001133 33 3310...1000000...
1401000100 01000100 0100010044 44 44000 100000...
1501010101 01010101 0101010155 55 55100... 000 1000...
1601100110 01100110 0110011066 66 660000 100000...000000 10000000...
1701110111 01110111 0111011177 77 77100010...
1810001000 10001000 1000100088 88 8800 100000...
1910011001 10011001 1001100199 99 990 100000...00 10000000...
2010101010 10101010 10101010AA AA AA00 100... 0 1000...
2110111011 10111011 10111011BB BB BB00 101000...
2211001100 11001100 11001100CC CC CC0 10...0000 10000000...
2311011101 11011101 11011101DD DD DD0 101000...
2411101110 11101110 11101110EE EE EE0 100010...
2511111111 11111111 11111111FF FF FF0 100...000 100000...
2610010010 01001001 0010010092 49 24 00 100000...0 100...
2701001001 00100100 1001001049 24 92 0000 100000...100 100...
2800100100 10010010 0100100124 92 49 100000...00 100...
2901101101 10110110 110110116D B6 DB 0 100...
3010110110 11011011 01101101B6 DB 6D 100...
3111011011 01101101 10110110DB 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.

Criticism

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 use software 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, including magnetic force microscopes, which together with image analysis, can detect the previous values of bits on the affected area of the media (for example hard disk).

Daniel Feenberg of the National Bureau of Economic Research, an American private nonprofit research organization, criticized Gutmann's claim that intelligence agencies are likely to be able to read overwritten data, citing a lack of evidence for such claims.[5] Nevertheless, some published government security procedures consider a disk overwritten once to still be sensitive.[6]

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][2]

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.
— Peter Gutmann, Secure Deletion of Data from Magnetic and Solid-State Memory, University of Auckland Department of Computer Science.

Software implementations

See also

Notes

  1. 1 2 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.
  2. 1 2 Lorrie Faith Cranor, Simson Garfinkel. Security and Usability: Designing Secure Systems that People Can Use. p. 307.
  3. ↑ Clearing and Declassifying Electronic Data Storage Devices (PDF) (PDF). Communications Security Establishment. July 2006. p. 7. Archived from the original (PDF) on 2014-03-03.
  4. ↑ https://www.usenix.org/legacy/events/fast11/tech/full_papers/Wei.pdf
  5. ↑ Daniel Feenberg (2013) [2003]. "Can Intelligence Agencies Read Overwritten Data? A response to Gutmann". National Bureau of Economic Research.
  6. ↑ "Clearing and Declassifying Electronic Data Storage Devices" (PDF) (PDF). Communications Security Establishment. July 2006. Archived from the original (PDF) on 2014-03-03.
  7. ↑ "Coreutils manual:shred, remove files more securely". Free Software Foundation. Retrieved 11 September 2012.

External links

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