Modern vehicles use Unified Diagnostic Services (UDS) or GMLAN (variation of CAN) for ECU reprogramming and parameter modification. To prevent unauthorized access, a seed‑key challenge‑response system is mandated by ISO 14229‑1 (SecurityAccess service $0x27$).
Think of it as a secret handshake: The ECU asks, "What is the code for number 42?" (Seed), and your tool must reply with the correct password (Key) based on a hidden math problem. Gm 5 Byte Seed Key