#pragma once
// IDS-CAN command-auth response cipher for the OneControl write path.
//
// 32-round TEA/XTEA-family Feistel, delta 0x9E3779B9, keyed by a per-session
// 32-bit "Cypher" with the round constants baked in.
// Verified bit-exact against ids_can_auth.py and 51 captured bus pairs.
//
// uint32_t arithmetic wraps mod 2^32 by definition in C++, so no masking needed.
// ESP32 int is 32-bit (uint32_t == unsigned int): host g++ test is representative.

#include <cstdint>
#include <cstddef>

namespace ids_can_auth {

// Per-session keys ("Cypher"). REMOTE_CONTROL gates on/off/move.
enum SessionKey : uint32_t {
  MANUFACTURING  = 0xB16BA115u,
  DIAGNOSTIC     = 0xBABECAFEu,
  REPROGRAMMING  = 0xDEADBEEFu,
  REMOTE_CONTROL = 0xB16B00B5u,
  DAQ            = 0x0B00B135u,
};

// response = Encrypt(challenge). seed = challenge word, cypher = session key.
inline uint32_t encrypt(uint32_t seed, uint32_t cypher) {
  uint32_t num = cypher;
  uint32_t sum = 0x9E3779B9u;  // TEA golden-ratio delta
  for (int rounds = 32;;) {
    seed += ((num << 4) + 1131376761u) ^ (num + sum) ^ ((num >> 5) + 1919510376u);
    if (--rounds <= 0) break;
    num  += ((seed << 4) + 1948272964u) ^ (seed + sum) ^ ((seed >> 5) + 1400073827u);
    sum  += 0x9E3779B9u;
  }
  return seed;
}

inline uint32_t remote_control_response(uint32_t challenge) {
  return encrypt(challenge, REMOTE_CONTROL);
}

// 4 big-endian challenge bytes (as they arrive in the page-42 payload, after the
// "00 04" prefix) -> 4 big-endian response bytes (for the page-43 reply).
inline void remote_control_response_bytes(const uint8_t challenge[4], uint8_t response[4]) {
  uint32_t c = (uint32_t)challenge[0] << 24 | (uint32_t)challenge[1] << 16 |
               (uint32_t)challenge[2] << 8  | (uint32_t)challenge[3];
  uint32_t r = remote_control_response(c);
  response[0] = (uint8_t)(r >> 24);
  response[1] = (uint8_t)(r >> 16);
  response[2] = (uint8_t)(r >> 8);
  response[3] = (uint8_t)(r);
}

}  // namespace ids_can_auth