Hard Disk Wiping methods
Information removed from a hard disk drive by non-secure means (for example, by simple Windows delete) can easily be recovered. Utilizing specialized equipment, it is possible to recover even repeatedly overwritten information.
Data is stored on a hard disk as a binary sequence of 1 and 0 (ones and zeros), represented by differently magnetized parts of a disk. Generally speaking, a 1 written to a hard disk is read as 1 by its controller, and 0 is read as 0. However, if you write 1 over 0, the result is conditionally 0.95 and vice versa – if 1 is written over 1 the result is 1.05. These differences are irrelevant for the controller. However, using special equipment, one can easily read the «underlying» sequence of 1's and 0's.
Information wiping methods
The detailed theory of guaranteed information wiping is described in an article by Peter Gutmann. See "Secure Deletion of Data from Magnetic and Solid-State Memory" at https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html.
| No. | Algorithm (writing method) | Passes | Record |
|---|---|---|---|
| 1. | United States Department of Defense 5220.22-M | 4 | 1 pass – randomly selected symbols to each byte of each sector, 2 – complementary to written during the first pass; 3 – random symbols again; 4 – writing verification. |
| 2. | United States: NAVSO P-5239-26 (RLL) | 4 | 1 pass – 0x01 to all sectors, 2 – 0x27FFFFFF, 3 – random symbol sequences, 4 – verification. |
| 3. | United States: NAVSO P-5239-26 (MFM) | 4 | 1 pass – 0x01 to all sectors, 2 – 0x7FFFFFFF, 3 – random symbol sequences, 4 – verification. |
| 4. | German: VSITR | 7 | Passes 1 – 6 – alternate sequences of: 0x00 and 0xFF; pass 7 – 0xAA; i.e. 0x00, 0xFF, 0x00, 0xFF, 0x00, 0xFF, 0xAA. |
| 5. | Russian: GOST P50739-95 | 1 |
Logical zeros (0x00 numbers) to each byte of each sector for the sixth to fourth security level systems. Randomly selected symbols (numbers) to each byte of each sector for the third to first security level systems. |
| 6. | Peter Gutmann's method | 35 | Peter Gutmann's method is very sophisticated. It's based on his theory of hard disk information wiping (see Secure Deletion of Data from Magnetic and Solid-State Memory). |
| 7. | Bruce Schneier's method | 7 | Bruce Schneier offers a seven-pass overwriting method in his Applied Cryptography book. 1 pass – 0xFF, 2 – 0x00, and then five times with a cryptographically secure pseudo-random sequence. |
| 8. | Fast | 1 | Logical zeros (0x00 numbers) to all sectors to wipe. |