The Mac BBS.
Since 1988.

Structurally identical to RSA. A 256-bit modulus N and key pair (P, S) where P×S≡1 mod φ(N). Only the author knew the private exponent P needed to generate serial numbers.

Arbitrary-precision integers up to 1024 bits, stored as arrays of 16-bit words. The critical inner loops — addition with carry, subtraction with borrow, schoolbook multiplication, and Knuth's Algorithm D for division — are all written in inline Motorola 68000 assembly using ADDX, SUBX, and MULU.W instructions. Modular exponentiation uses right-to-left binary square-and-multiply.

Classic Mac OS used cooperative multitasking — a BBS serving multiple callers couldn't block the event loop for RSA computation. The solution: StepPowerModBigInts, an incremental square-and-multiply that advanced a few iterations per call. Validation ran continuously in the background during normal I/O, completing the cryptographic proof over hundreds of event loop cycles.

Failed validation didn't display an error — it used a custom setjmp/longjmp in raw 68000 machine code to redirect program flow. The machine code was stored as a checksummed integer array and executed via obfuscated indirect jump. A pirated copy appeared to work but degraded subtly — skipped session init, silent message failures — making it hard for crackers to confirm defeat.