Your tool sends a request (e.g., 27 01 ) to the ECU. The ECU responds with a 5-byte Seed —a random string of numbers that acts as a one-time question.
GM’s 5-byte seed/key algorithm is a classic example of security-by-obscurity in automotive ECUs. It was sufficient to deter casual users but trivial for determined reverse engineers. Its widespread documentation now enables legitimate aftermarket repairs, tuning, and salvage module reprogramming. However, it should never be used in new designs, and indeed GM has since moved to stronger methods.
The GM 5-Bit algorithm is classified as .
