The Gutmann method is an algorithm for securely erasing the contents of computer hard drives, such as files. In Mathematics, Computing, Linguistics and related subjects an algorithm is a sequence of finite instructions often used for Calculation Data remanence is the residual representation of Data that has been in some way nominally erased or removed A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device A computer file is a block of Arbitrary Information, or resource for storing information which is available to a Computer program and is usually Devised by Peter Gutmann and Colin Plumb, it does so by writing a series of 35 patterns over the region to be erased. Peter Gutmann is a computer scientist in the Department of Computer Science at the University of Auckland, Auckland, New Zealand. A pattern, from the French patron, is a theme of recurring events or objects sometimes referred to as elements of a set
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. This article is about Modified Frequency Modulation For other uses of MFM see the MFM (disambiguation Run length limited or RLL coding is a technique that is used to store data on recordable media Relatively modern drives no longer use the older encoding techniques, making many of the patterns specified by Gutmann 'superfluous'. 
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.
Analog signal: +11. 1 -8. 9 +9. 1 -11. 1 +10. 9 -9. 1Ideal 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. 9Previous 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 +9Ideal Digital signal: +10. 0 +10. 0 -10. 0 -10. 0 +10. 0 +10. 0 Difference: +1 +1 +1 -1 -1 -1Previous signal: +10 +10 -10 -10 -10 -10
In 1996, when this method was developed, it was possible to use a digital oscilloscope to recover eight levels of overwrites, without damaging the drive. An oscilloscope (commonly abbreviated to scope or O-scope) is a type of Electronic test equipment that allows signal Voltages to be viewed Since then higher disk densities have probably reduced the number of overwrites necessary to completely erase data.
However, overwriting the disk repeatedly with random data will not always work. The permittivity of a medium changes with the frequency of the magnetic field. Permittivity is a Physical quantity that describes how an Electric field affects and is affected by a Dielectric medium and is determined by the ability 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 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.
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. The Insert key (often abbreviated INS) is a key commonly found on Computer keyboards It is primarily used to switch between the two text-entering modes on Randomness is a lack of order Purpose, cause, or predictability
Each of patterns 5-31 was designed with a specific magnetic media encoding scheme in mind, which each pattern targets. Magnetic storage and magnetic recording are terms from Engineering referring to the storage of Data on a Magnetized medium In Communications a code is a rule for converting a piece of Information (for example a letter, Word, Phrase, or 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. An Atomic force microscopy (AFM Unlike typical AFM magnetic materials are used for the sample and tip so that not only the atomic force but also the magnetic interaction are detected
The series of patterns is as follows:
|Pass||Data Written||Pattern written to disk for targeted encoding scheme|
|In Binary notation||In Hex notation||(1,7) RLL||(2,7) RLL||MFM|
|5||01010101 01010101 01010101||55 55 55||100. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. In Mathematics and Computer science, hexadecimal (also base -, hexa, or hex) is a Numeral system with a Run length limited or RLL coding is a technique that is used to store data on recordable media Run length limited or RLL coding is a technique that is used to store data on recordable media This article is about Modified Frequency Modulation For other uses of MFM see the MFM (disambiguation . .||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. . .|
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.
Some have criticized Gutmann's claim that intelligence agencies are likely to be able to read overwritten data. 
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. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination 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). An intelligence agency is a governmental agency that is devoted to the Information gathering (known in the context as " intelligence " An Atomic force microscopy (AFM Unlike typical AFM magnetic materials are used for the sample and tip so that not only the atomic force but also the magnetic interaction are detected Image analysis is the extraction of meaningful information from Images mainly from Digital images by means of Digital image processing techniques A bit is a binary digit, taking a value of either 0 or 1 Binary digits are a basic unit of Information storage and communication A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device
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. 
Companies specializing in recovery from damaged media cannot recover completely overwritten files. Data recovery is the process of salvaging data from damaged failed corrupted or inaccessible secondary storage media when it cannot be accessed normally 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:
|“||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 Computer data storage, Partial Response Maximum Likelihood (PRML is a method for converting the weak Analog signal from the head of a magnetic disk or tape drive In Magnetic storage for computers extended partial response maximum likelihood, also known as extended PRML or EPRML, is an evolutionary improvement on 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). An Atomic force microscopy (AFM Unlike typical AFM magnetic materials are used for the sample and tip so that not only the atomic force but also the magnetic interaction are detected 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.||”|
Bad sectors on the disk may be silently suppressed by the drive controller so they may not be overwritten.