Performing a "master cylinder bleeding" or "lateral accelerometer calibration" on GM vehicles often requires unlocking the ABS module via a 5 Byte Seed Key.
: Each ECU vendor may have a unique security table with multiple algorithm rows, making it harder for a single "master key" to exist. Gm 5 Byte Seed Key ((hot)) Gm 5 Byte Seed Key
To understand the 5 Byte variant, one must first grasp the concept of Unified Diagnostic Services (UDS) and seed/key exchange in general. Gm 5 Byte Seed Key