Copylock
The protection standard
Rob Northen's disk protection system that became the industry standard on Amiga and Atari ST, using sophisticated techniques to detect original disks and resist cracking.
Overview
Copylock was the dominant disk copy-protection system for the Amiga and Atari ST, created by Rob Northen Computing. Used on hundreds of commercial games, it employed sophisticated techniques including non-standard disk encoding, rotational-timing checks, and encrypted verification code that decrypted in memory only when the disk's physical signature checked out. While each version was eventually cracked, Copylock represented the state of the art in 16-bit home-computer protection.
Fast facts
- Creator: Rob Northen Computing — also responsible for the Spectrum's Speedlock.
- Platforms: Commodore Amiga, Atari ST.
- Era: 1987-mid 1990s.
- Usage: Hundreds of commercial games — many top-tier UK and European releases.
- Evolution: Multiple major versions, each tightening the previous.
- Status: Ultimately always cracked, but each version bought months of protection.
How it worked
Copylock combined multiple layers, any one of which could detect a copy:
| Layer | Purpose |
|---|---|
| Physical disk encoding | Non-standard MFM patterns: weak bits, fuzzy bits, long sectors |
| Rotational timing | Measure index-pulse intervals against expected disk-rotation profile |
| Sync-mark detection | Custom sync-mark patterns at non-standard track positions |
| Encrypted verification code | The protection routine itself is decrypted in memory using the disk-derived key |
| Anti-debug | Detects single-stepping, breakpoints, trace bits |
| Integrity checks | The protection code self-checksums to detect static patches |
The rotation-timing trick
The signature Copylock check: read a specific sector, then read it again on the next rotation. Measure the time between reads. A genuine disk produces a rotation time consistent with the manufacturer's spindle motor; a copied disk written on a different drive will produce slightly different timing because its tracks are placed slightly differently radially. The check has microsecond resolution and rejects all but a properly mastered original.
This is hard to beat because it isn't checking a value — it's checking the disk's physical motion against expectations.
Verification process
- Load the encrypted protection block from the disk.
- Read the disk's "key" track — a track with weak bits, long sectors, or other physical anomalies.
- Derive a decryption key from the bit-pattern of the key track.
- Decrypt the verification code in RAM using that key. If the disk is wrong, the decryption produces garbage instead of valid 68000 instructions.
- Run the verification code — measure timing, check additional sync marks, verify multiple physical anomalies.
- Either return to the game's main routine or crash deliberately on failure.
The crucial detail: the game's actual code is encrypted using the same key. A cracker who NOPs out the protection check still has an encrypted game.
Evolution
| Version | Year | Innovation |
|---|---|---|
| Copylock I | 1987-88 | Basic weak-bit and sync-mark checks |
| Copylock II | 1989 | Encrypted verification routines |
| Copylock III | 1990-91 | Multiple checks, anti-debug, self-checksumming |
| Copylock IV | 1992-93 | Sophisticated evasion (jitter detection, encrypted decryptor) |
| Copylock V | 1993-95 | Final iteration; very nearly the end of disk-based protection |
Each version raised the cracker bar for several months until the demoscene's reverse-engineers caught up.
Why it dominated
Publishers chose Copylock because:
- Track record — proven effective for many months per version.
- Professional service — Rob Northen Computing offered integration support.
- Updates — when a version was cracked, RNC released the next.
- Reputation — became the industry default; not using Copylock was a marker of either bargain-basement or DRM-free philosophy.
The cracker response
Copylock forced the cracking scene to mature:
- Better disassemblers — handling the Copylock encrypted-instruction decoder required tools beyond the era's standard hex-editor work.
- Timing emulators — trace-and-replay drives that could fake rotation timing for testing.
- Collaborative reverse-engineering — single crackers couldn't keep up; groups (Skid Row, Quartex, Defjam) emerged.
- Custom tools — group-internal RNC-defeat tools that propagated through the scene over months.
Many demoscene names (Yates, Triad, Razor 1911) cut their teeth on Copylock and went on to legitimate careers in security or game development.
Preservation impact
Copylock-protected disks pose a specific archival challenge:
- Standard disk-image tools fail — the protection's whole point is producing data normal tools can't capture.
- Special hardware required — KryoFlux, SuperCard Pro, and similar flux-imaging tools capture the raw flux transitions and preserve the physical signal.
- Often preserved via cracks — cracked, deprotected versions are easier to archive; a faithful preservation needs the original disk and the special hardware.
- Emulator challenge — accurate disk-rotation emulation is required to play unmodified Copylock images. WinUAE supports this; many older Amiga emulators don't.
Legacy
Copylock was the most successful commercial protection of its 16-bit era — not because it was unbreakable, but because it consistently raised the bar. It professionalised both protection development and cracking, creating the technical challenges that pushed scene members toward legitimate careers. Many ex-Copylock-era reverse engineers now work in cybersecurity, anti-cheat, and DRM development at studios; the same skill set, applied legally.