I am building a diagnostic app for a Ford Everest 2015 2.2L Duratorq diesel. I am trying to reverse engineer the seed-to-key algorithm used by the PCM (UDS Service 0x27 Security Access).

**Setup:**

- PCM TX: 0x7E0 / RX: 0x7E8 (HS-CAN 500k)

- Session: 10 02 (Programming session)

- Seed is 3 bytes, key must also be 3 bytes

- Frame format: `05 27 02 KK KK KK 00 00`

**Seed/key pairs captured by sniffing FORScan:**

```

Seed: 3C 39 40 → Key: ?? ?? ??

Seed: 47 B8 4F → Key: ?? ?? ??

Seed: AE 84 7C → Key: ?? ?? ??

```

*(I am still capturing key values using a CAN sniffer while FORScan performs security access — will update pairs as soon as captured)*

**What I have tried:**

- Simple XOR with constant mask — no consistent pattern yet

- Bit rotation then XOR — cannot confirm without key values

- LFSR — not enough data yet

**My question:**

Once I have full seed+key pairs — what is the best approach to reverse engineer a 3-byte automotive seed-to-key algorithm? Are there known patterns used by Ford 2015 PCMs? Is there open source tooling that can help analyze pairs and identify the formula?

Any experience with Ford Ranger/Everest/Transit 2013-2016 PCM security access algorithms is appreciated.