Secure deletion method

Gutmann’s 35-Pass Method

Gutmann’s method overwrites a file 35 times with a fixed sequence of random data and bit patterns, so that the original data cannot be recovered even with hardware-level analysis of the disk surface. Advanced File Shredder offers it as its most thorough erasure method.

Passes

35

Source

Peter Gutmann, 1996

Best for

Maximum assurance on magnetic hard drives

Where the method comes from

The method is based on Peter Gutmann’s paper “Secure Deletion of Data from Magnetic and Solid-State Memory”, section “Erasure of Data stored on Magnetic Media”. The paper shows that traces of earlier data can remain on a magnetic disk after it has been overwritten, and designs a sequence of 35 passes that destroys those traces regardless of how the drive encodes data on its platters.

The 35 passes

Passes 1–4 and 32–35 write random data. Passes 5–31 write the patterns below, each repeated across the whole file. Patterns made of three bytes repeat every three bytes.

Should you use 35 passes?

Gutmann designed the sequence to cover several older drive encoding schemes at once, so many of the passes target encodings a particular drive does not use. For modern hard drives, a few passes of cryptographically strong random data or the 7-pass US DoD 5220.22-M method are much faster and are enough in practice. Use Gutmann’s method when you want the strongest option and erasure time does not matter.

Advanced File Shredder reads the last pass back from the disk to verify that the data was really overwritten, whichever method you choose. It also overwrites NTFS alternate data streams, not just the main file contents.