Key Management & Distribution

#!/usr/bin/env python3
# EOL production key provisioning simulation
import hmac, hashlib, struct

def derive_esk(root_key: bytes, vin: str,
               ecu_id: str, bus: str, direction: str) -> bytes:
    vms = hmac.new(b"automotive-v1", root_key, hashlib.sha256).digest()
    ctx = f"{ecu_id}|{bus}|{direction}".encode()
    return hmac.new(vms, ctx, hashlib.sha256).digest()[:16]

def make_she_m1(ecu_id: int, key_slot: int, counter: int) -> bytes:
    # M1: ECU_ID[4] + KEY_SLOT[2] + counter[4] -- prevents key replay
    return struct.pack(">IHI", ecu_id, key_slot, counter)

root_key = bytes.fromhex("deadbeef" * 4)
vin = "WBA12345678901234"

esk = derive_esk(root_key, vin, "TCU", "CAN0", "TX")
m1  = make_she_m1(ecu_id=0xA300, key_slot=0x01, counter=1)

print(f"VIN: {vin}")
print(f"TCU CAN0 TX ESK: {esk.hex()}")
print(f"SHE M1: {m1.hex()}")
print()
print("Provisioning flow:")
print("  1. KMS generates M1/M2/M3 using MASTER_ECU_KEY for this ECU")
print("  2. Provisioning station sends M1-M3 via SHE CMD_LOAD_KEY over SPI/FlexRay diag")
print("  3. SHE verifies M3 (CMAC auth proof) before loading key into KEY_1 slot")
print("  4. SHE returns M4+M5 (provisioning receipt)")
print("  5. KMS archives M4+M5 -- proves correct key was loaded in this ECU serial number")
print("  6. Production station locks KEY_1 slot: SET_WRITE_PROTECTION (irreversible)")

/* AUTOSAR KeyM: receive updated SecOC key via OTA + activate atomically */
#include "KeyM.h"
#include "NvM.h"

/* Flow:
   OEM backend encrypts new ESK with current ECU master key
   → packages as KeyM UpdateRequest
   → delivers via UDS TransferData 0x36 over TLS DoIP
   → ECU KeyM: decrypt, verify anti-rollback, store pending, activate on next init */

Std_ReturnType KeyM_UpdateSecOCKey(const uint8* enc_new_key, uint8 new_version)
{
    uint8 new_key[16];

    /* Step 1: Decrypt new key using current master key in HSM slot */
    if (KeyM_DecryptKey(KEYM_SLOT_MASTER, enc_new_key, new_key) != KEYM_E_OK)
        return E_NOT_OK;

    /* Step 2: Anti-rollback: new version must be >= current */
    uint8 cur_version = NvM_ReadByte(NVM_BLOCK_SECOC_KEY_VERSION);
    if (new_version < cur_version) {
        Dem_ReportErrorStatus(DEM_EVENT_KEYM_ROLLBACK, DEM_EVENT_STATUS_FAILED);
        return E_NOT_OK;
    }

    /* Step 3: Store in pending slot; atomically activated on next ignition cycle */
    NvM_WriteBlock(NVM_BLOCK_SECOC_KEY_PENDING, new_key);
    NvM_WriteByte(NVM_BLOCK_SECOC_KEY_VERSION,  new_version);
    KeyM_ScheduleActivation(KEYM_SLOT_SECOC_PENDING, KEYM_ACTIVATE_ON_NEXT_INIT);

    return E_OK;
}