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Gutmann-Methode
In brief
The Gutmann method is an overwriting method in which a hard drive is written in 35 passes with fixed and random patterns. It goes back to a technical paper by Peter Gutmann from 1996 and was tailored to the recording methods of hard drives at the time. For today's hard drives it is considered unnecessarily laborious, for SSDs unsuitable.
Where the 35 passes come from
Gutmann investigated how data on magnetic data carriers might possibly be recovered after erasure. The hard drives of the time used different encoding methods. The 35 patterns were meant to cover all of these methods because it was not known which one a drive used. For a single hard drive, therefore, only part of the passes was ever useful. Gutmann himself later pointed out that for modern drives a few passes with random data suffice.
| Method | Passes |
|---|---|
| Simple overwriting | 1 |
| DoD 5220.22-M (common variant) | 3 |
| VSITR (former BSI standard) | 7 |
| Gutmann method | 35 |
Why more passes are not more secure
The US guideline NIST SP 800-88 states that multiple overwriting offers hardly any protection for SSDs with spare blocks. Software does not reach all memory cells there. For HDDs, hidden areas such as HPA and DCO and reallocated sectors decide security more than the number of passes. More on this in the entry Overwriting and in the guide Securely erasing hard drives and SSDs. Second IT erases with Blancco, captures hidden areas in the process and documents every erasure with a certificate per device.
Frequently asked questions about the Gutmann method
Is the Gutmann method the most secure erasure method?
No. It is the most laborious, but not the most secure. On modern hard drives additional passes bring no measurable gain, and for SSDs it is unsuitable.
How long does the Gutmann method take?
Very long on large hard drives, because every sector is written 35 times. With today's capacities this can take days.
Does the Gutmann method harm an SSD?
It stresses the memory cells through many write operations and still does not reach all areas. For SSDs, Sanitize commands or Crypto Erase are the better methods.
Why does the Gutmann method have 35 passes?
The hard drives of the time used different encoding methods. The 35 patterns were meant to cover all of these methods because it was not known which one a drive used. For a single hard drive, therefore, only part of the passes was ever useful.
How many passes do DoD 5220.22-M and VSITR have?
DoD 5220.22-M is used in the common variant with 3 passes, VSITR as a former BSI standard with 7. Simple overwriting has one pass, the Gutmann method 35. More passes do not automatically mean more security.
Thorough instead of cumbersome: erasure with a certificate per device.
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