# uds_sim.py - ECU that authenticates with a simple challenge
import random, hashlib

class ECU:
    def __init__(self):
        self.secret = b'long_secret_key'
    def generate_challenge(self):
        self.challenge = random.randbytes(8)
        return self.challenge
    def verify_response(self, response):
        expected = hashlib.sha256(self.secret + self.challenge).digest()[:8]
        return response == expected
    def unlock(self):
        print("ECU unlocked! Critical functions accessible.")

# Attacker captures a valid challenge-response pair
ecu = ECU()
challenge = ecu.generate_challenge()
# Legitimate tool computes response (simplified)
response = hashlib.sha256(ecu.secret + challenge).digest()[:8]
ecu.verify_response(response)  # first unlock

# Replay the same challenge-response
ecu.challenge = challenge
ecu.verify_response(response)  # second unlock without new challenge - works
ecu.unlock()

python uds_sim.py