void gm5_compute_key(const uint8_t seed[5], uint8_t key[5]) seed[1]; for (int i = 0; i < 5; i++) uint8_t idx = (state >> 8) ^ seed[i]; key[i] = gm5_table[idx]; state += key[i];
| Variant | Table | Initial state | Key generation order | |--------------------|---------------|-----------------------------|----------------------------| | | LFSR 0x3FF | (seed[0]<<8)|seed[1] | Forward (i=0..4) | | Global A (E92) | Same | (seed[4]<<8)|seed[3] | Reverse (i=4..0) | | Theta platform | Modified table (poly x^10 + x^8 + 1) | 0x155 | Forward but seed permuted | Gm 5 Byte Seed Key
def generate_full_table(): lfsr = 0x3FF table = [] for _ in range(256): table.append(lfsr & 0xFF) bit = ((lfsr >> 9) & 1) ^ ((lfsr >> 4) & 1) ^ ((lfsr >> 2) & 1) ^ ((lfsr >> 1) & 1) lfsr = ((lfsr << 1) | bit) & 0x3FF return table print([hex(b) for b in generate_full_table()]) void gm5_compute_key(const uint8_t seed[5]
Most aftermarket tools implement the and switch based on ECU ID. 5. Implementation in C (J2534‑Ready) #include <stdint.h> extern const uint8_t gm5_table[256]; // precomputed uint8_t key[5]) seed[1]
def gen_gm_table(): lfsr = 0x3FF table = [] for i in range(256): table.append(lfsr & 0xFF) # 10-bit LFSR step bit = ((lfsr >> 9) ^ (lfsr >> 4) ^ (lfsr >> 2) ^ (lfsr >> 1)) & 1 lfsr = ((lfsr << 1) | bit) & 0x3FF return table uint8_t key[5]; uint16_t state = (seed[0] << 8) | seed[1]; uint8_t idx; for (int i = 0; i < 5; i++) idx = (state >> 8) ^ seed[i]; key[i] = gm_table[idx]; state = (state + key[i]) & 0xFFFF;