Western Security fingerprint-coded BBC Micro floppies so copiers could not clone them

BBC Micro software houses faced a copying wave: general copying programs such as Disc Duplicator 3 made most protected floppies easy to clone, because exotic protection usually just meant unusual sectors that a dedicated copying program could reproduce with a standard controller. Western Security Ltd, a small company George Keeling set up in the west of England, wanted protection a normal home computer could author but no normal disc drive could duplicate.

Their answer, which the firm called fingerprinting and which Keeling credits to Bill Cawley, turned the disc's analog physics into the secret. Formatting fills each track with padding bytes, and how many padding bytes fit depends on the exact speed of the drive doing the formatting, which varied from drive to drive. Western Security formatted each disc, measured the resulting lengths of tracks 1–8, then wrote that disc's expected-length table into its own loader on track 9, buried under layers of self-modifying, obfuscated code.

At load time the game read every full track and demanded that 7 or 8 of the 8 measured lengths land within ±1 byte of the stored table — tolerance that lets a genuine disc pass on any drive while rejecting anything cloned elsewhere. The scheme ran on both BBC Micro controller families through completely different code paths, and the eight fingerprint tracks looked perfectly ordinary: correct sectors, no exotic timing, nothing an analysis tool would flag.

Copies failed because track length is not data: a duplicating drive spinning at its own speed writes its own slightly different lengths, and even the dedicated duplication hardware of the era struggled to hit byte-perfect matches. Two original copies of the same title carried different fingerprints, confirming each disc was personalized at mastering. In a 2020 audit of nearly every copy-protected BBC Micro disc, security researcher Chris Evans called this the most powerful protection he found — protection that made recreation, not writing, the hard step.

Make each disc's drive-speed fingerprint its own key

Creation costs almost nothing: a plain format produces the fingerprint, so publishers needed no exotic mastering machines or special write patterns.

The fingerprint is invisible to inspection: the protected tracks are ordinary sectors, so analysis tools and copiers see nothing unusual to work around.

It cannot be reproduced by a different drive: track length records the formatting drive's spindle speed, and cloning demands byte-perfect lengths from a drive spinning at the same speed.

Tolerance keeps genuine discs working: ±1 byte with a 7-of-8 pass rate absorbs read-to-read analog noise across drives and controllers.

Every disc is unique: expected lengths are generated per disc after formatting, so no master table exists for a cracker to lift once.

When copies are cheap, authenticate against an analog property that is nearly free to stamp at creation yet almost impossible to reproduce: make recreation, not writing, the hard step.

Owner George Keeling later said the system was not reliable enough, and Sherston programmer Simon Hosler recalled that it became a pain as disc drives kept changing; Phantom Combat even shipped with a sticker warning that only Acorn disc ROMs were supported. Surviving discs date to 1985–1986, and the scheme faded with the BBC Micro era. Decades later researcher Chris Evans, auditing nearly every copy-protected BBC Micro disc, called it the most powerful protection he found and the cleverest floppy protection he had seen, and Keeling confirmed the scheme in the comments.