Gutmann method

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). This claim however seems to be invalid based on the thesis "Data Reconstruction from a Hard Disk Drive using Magnetic Force Microscopy".{{cite web|url=https://escholarship.org/uc/item/26g4p84b|title=Data Reconstruction from a Hard Disk Drive using Magnetic Force Microscopy|archiveurl=https://web.archive.org/web/20151027143757/https://escholarship.org/uc/item/26g4p84b|archivedate=2015-10-27|publisher=UNIVERSITY OF CALIFORNIA, SAN DIEGO|date=2013|type=PDF}}

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:

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.

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. He finds that Gutmann cites one non-existent source and sources that do not actually demonstrate recovery, only partially-successful observations. The definition of "random" is also quite different from the usual one used: Gutmann expects the use of pseudorandom data with sequences known to the recovering side, not an unpredictable one such as a cryptographically secure pseudorandom number generator.{{cite web|url=http://www.nber.org/sys-admin/overwritten-data-gutmann.html|title=Can Intelligence Agencies Read Overwritten Data? A response to Gutmann|publisher=National Bureau of Economic Research|date=2013|orig-year=2003|author=Daniel Feenberg}}

Nevertheless, some published government security procedures consider a disk overwritten once to still be sensitive.{{cite web|url=http://www.cse-cst.gc.ca/documents/publications/itsg-csti/itsg06-eng.pdf|title=Clearing and Declassifying Electronic Data Storage Devices|archiveurl=https://web.archive.org/web/20140303210956/http://www.cse-cst.gc.ca/documents/publications/itsg-csti/itsg06-eng.pdf|archivedate=2014-03-03|publisher=Communications Security Establishment|date=July 2006|type=PDF}}

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:Gutmann, Peter. (July 22–25, 1996) [https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html Secure Deletion of Data from Magnetic and Solid-State Memory.] University of Auckland Department of Computer Science. Epilogue section.

{{quotation|text=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.|author=Peter Gutmann|title=Secure Deletion of Data from Magnetic and Solid-State Memory|source=University of Auckland Department of Computer Science}}

Gutmann's statement has been criticized for not recognizing that PRML/EPRML does not replace RLL, with critics claiming PRML/EPRML to be a signal detection method rather than a data encoding method. Polish data recovery service [https://kaleron.edu.pl/ Kaleron] has also claimed that Gutmann's publication contains further factual errors and assumptions that do not apply to actual disks.{{cite web|date=2024|title=Throwing Gutmann's algorithm into the trash|author=Kaleron|url=https://kaleron.edu.pl/throwing-Gutmanns-algorithm-into-the-trash|access-date=2024-11-22}}

• Data remanence
• Data recovery
• Computer forensics

{{Reflist}}

• [https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html Secure Deletion of Data from Magnetic and Solid-State Memory], Gutmann's original paper

{{Data Erasure}}

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Category:Data erasure

Category:Algorithms