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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 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".〔 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. However, a 2011 research found that 4 out of 8 manufacturers did not implement ATA Secure Erase correctly.〔https://www.usenix.org/legacy/events/fast11/tech/full_papers/Wei.pdf〕 ==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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gutmann method」の詳細全文を読む スポンサード リンク
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