Unlike modern cryptography (like RSA or AES), automotive seed-key algorithms are typically lightweight, obfuscated logic operations. They often consist of:
A generic pseudo-code representation of a GM-style algorithm might look like this:
// Simplified conceptual logic // Input: 5-byte Seed // Output: 5-byte Keyuint8_t seed[5] = ... ; uint8_t key[5];
// The algorithm usually applies a specific transformation logic // for each byte, often dependent on the previous byte. key[0] = seed[0] ^ SECRET_MASK_A; key[1] = (seed[1] + seed[0]) ^ SECRET_MASK_B; // ... and so ongm 5 byte seed key
In reality, GM algorithms are often slightly more complex, involving bitwise rotations and specific constants found in the firmware.
Automakers operate against a landscape of constraints: real‑time responsiveness, limited ECU RAM/flash, and years‑old protocols that predate contemporary threat models. A five‑byte seed cuts computational cost, reduces message size, and stays compatible with older tooling—practical incentives when you’re shipping millions of vehicles and patching hardware post‑sale is costly and slow. Unlike modern cryptography (like RSA or AES), automotive
Despite its complexity, the GM 5 byte seed key had fatal architectural flaws:
Used in Body Control Modules for the Immobilizer system (Passlock III / PK3+).
For many GM ECMs (2010–2018):
Key[0] = (Seed[0] * 0x4D + 0x6A) ^ Seed[1]
Key[1] = (Seed[1] * 0x4D + 0x6A) ^ Seed[2]
Key[2] = (Seed[2] * 0x4D + 0x6A) ^ Seed[3]
Key[3] = (Seed[3] * 0x4D + 0x6A) ^ Seed[4]
Key[4] = (Seed[4] * 0x4D + 0x6A) ^ Seed[0]
All operations mod 0x100 (byte arithmetic).
Constants 0x4D (77 decimal) and 0x6A (106 decimal) are common but not universal.
Let’s walk through a real-world use case:
Vehicle: 2010 Chevrolet Equinox (BCM controlled immobilizer) Task: Add a new transponder key (PK3+) A generic pseudo-code representation of a GM-style algorithm
If step 6 fails (reply is 7F 27 35 or 7F 27 36), the key was invalid. You likely used the wrong algorithm variation.
