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The SARSDP-23 Secure Erase Algorithm

In this article I’ll try to explain the logic behind the Rizonesoft file overwriting method (algorithm) called the South African RSDP-23 method or SARSDP-23 for short. I will explain why it was necessary to develop another file overwriting method and my personal thoughts about overwriting methods, like the one developed by Peter Gutmann. I will also give you some information about the specific data encoding scheme SARSDP-23 targets, namely the popular Run Left Limited (RLL) encoding scheme.

Quick Erase Options (SARSDP-23)

The clues are in the name

The naming convention I used for Rizonesoft’s overwriting method is simple to understand. SA stands for South African, RS is for Rizonesoft, DP is for Derick Payne (me) and 23 indicates the number of passes (the amount of times a file will be overwritten)

Run Left Limited Encoding

RLL encoding was invented by IBM and first used the method in many of its mainframe disk drives. During the late 1980s, the PC hard disk industry began using RLL encoding schemes to increase the storage capabilities of PC hard disks. Most modern hard disks use RLL encoding.

Instead of encoding a single bit, RLL typically encodes a group of data bits at a time. The term Run Length Limited is derived from the two primary specifications of these codes, which are the minimum number (the run length) and maximum number (the run limit) of transition cells allowed between two actual flux transitions. Several variations of the scheme are achieved by changing the length and limit parameters, but only two have achieved real popularity: RLL 2,7 and RLL 1,7.

Initially RLL 2,7 was the most popular RLL variation. This method provides high storage density and good reliability. However, in high-capacity drives it did not prove to be reliable enough. Most of today’s hard drives use RLL 1,7 encoding. Because of the larger relative timing window or cell size within which a transition can be detected, RLL 1,7 is a more forgiving and more reliable code, which is important when media and head technology are being pushed to their limits. The SARSDP-23 method targets RLL 1,7 encoding, because it makes sense to mainly target a modern data encoding scheme.

SARSDP-23 vs Other overwriting methods

Overwriting methods, like Gutmann targets some obsolete encoding schemes and this results in unnecessary passes. SARSDP-23 targets only modern encoding schemes and this makes it faster and not as resource intensive. Some other method is an extreme overkill where SARSDP-23 is just overkill. SARSDP-23 hides the fact that it’s been used to overwrite a file with 0s in the last pass. 0s is the default (clean state) for hard disks.

Don’t get me wrong, I love the Gutmann and other methods, I just think that they are not optimized for modern disk drives and a new algorithm is long overdue.

SARSDP-23 patterns

The table below lists each pass of the SARSDP-23 method and give you some information about the data being written and the targeted encoding scheme. Note that the random and compliment passes do not target a specific encoding scheme.


PassHex NotationBinary NotationTargeted Encoding
 1 (Random) (Random) –
 2 (Compliment) (Compliment) –
 3 (Random) (Random) –
 4 0x00 00000000 RLL 1,7
 5 0x11 00010001 RLL 1,7
 6 0x22 00100010 RLL 1,7
 7 0x33 00110011 RLL 1,7
 8 0x44 01000100 RLL 1,7
 9 0x55 01010101 RLL 1,7
 10 0x66 01100110 RLL 1,7
 11 0x77 01110111 RLL 1,7
 12 0x88 10001000 RLL 1,7
 13 0x99 10011001 RLL 1,7
 14 0xAA 10101010 RLL 1,7
 15 0xBB 10111011 RLL 1,7
 16 0xCC 11001100 RLL 1,7
 17 0xDD 11011101 RLL 1,7
 18 0xEE 11101110 RLL 1,7
 19 0xFF 11111111 RLL 1,7
 20 (Random) (Random) –
 21 (Compliment) (Compliment) –
 22 (Random) (Random) –
 23 00 RLL 1,7

I hope this article shed some light on the South African RSDP-23 file overwriting algorithm (method).

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