ble_aes_ccm.cpp

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#include "ble_aes_ccm.h"
 
#include <algorithm>
#include <cstring>
 
namespace esphome::ble_device_base {
 
namespace {
 
// AES-128 forward cipher only — CCM uses the block cipher in the encrypt
// direction for both the CTR keystream and the CBC-MAC.
const uint8_t SBOX[256] = {
    0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe, 0xd7, 0xab, 0x76,  //
    0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4, 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0,  //
    0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7, 0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15,  //
    0x04, 0xc7, 0x23, 0xc3, 0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2, 0xeb, 0x27, 0xb2, 0x75,  //
    0x09, 0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0, 0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3, 0x2f, 0x84,  //
    0x53, 0xd1, 0x00, 0xed, 0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe, 0x39, 0x4a, 0x4c, 0x58, 0xcf,  //
    0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85, 0x45, 0xf9, 0x02, 0x7f, 0x50, 0x3c, 0x9f, 0xa8,  //
    0x51, 0xa3, 0x40, 0x8f, 0x92, 0x9d, 0x38, 0xf5, 0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2,  //
    0xcd, 0x0c, 0x13, 0xec, 0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19, 0x73,  //
    0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14, 0xde, 0x5e, 0x0b, 0xdb,  //
    0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c, 0xc2, 0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79,  //
    0xe7, 0xc8, 0x37, 0x6d, 0x8d, 0xd5, 0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08,  //
    0xba, 0x78, 0x25, 0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f, 0x4b, 0xbd, 0x8b, 0x8a,  //
    0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e, 0x61, 0x35, 0x57, 0xb9, 0x86, 0xc1, 0x1d, 0x9e,  //
    0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e, 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf,  //
    0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42, 0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16,  //
};
 
const uint8_t RCON[11] = {0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36};
 
inline uint8_t xtime(uint8_t x) { return static_cast<uint8_t>((x << 1) ^ ((x & 0x80) ? 0x1b : 0x00)); }
 
// AES-128 forward cipher with on-the-fly key schedule.
class Aes128 {
 public:
  explicit Aes128(const uint8_t key[16]) {
    memcpy(this->rk_, key, 16);
    for (size_t i = 16; i < 176; i += 4) {
      uint8_t t[4] = {this->rk_[i - 4], this->rk_[i - 3], this->rk_[i - 2], this->rk_[i - 1]};
      if (i % 16 == 0) {
        const uint8_t tmp = t[0];
        t[0] = static_cast<uint8_t>(SBOX[t[1]] ^ RCON[i / 16]);
        t[1] = SBOX[t[2]];
        t[2] = SBOX[t[3]];
        t[3] = SBOX[tmp];
      }
      for (size_t j = 0; j < 4; j++)
        this->rk_[i + j] = static_cast<uint8_t>(this->rk_[i - 16 + j] ^ t[j]);
    }
  }
 
  void encrypt(const uint8_t in[16], uint8_t out[16]) const {
    uint8_t s[16];
    memcpy(s, in, 16);
    for (size_t i = 0; i < 16; i++)
      s[i] ^= this->rk_[i];
 
    for (size_t round = 1; round < 10; round++) {
      for (uint8_t &b : s)
        b = SBOX[b];
      shift_rows(s);
      for (size_t c = 0; c < 4; c++) {
        uint8_t *col = s + c * 4;
        const uint8_t a0 = col[0], a1 = col[1], a2 = col[2], a3 = col[3];
        const uint8_t h = static_cast<uint8_t>(a0 ^ a1 ^ a2 ^ a3);
        col[0] ^= static_cast<uint8_t>(h ^ xtime(static_cast<uint8_t>(a0 ^ a1)));
        col[1] ^= static_cast<uint8_t>(h ^ xtime(static_cast<uint8_t>(a1 ^ a2)));
        col[2] ^= static_cast<uint8_t>(h ^ xtime(static_cast<uint8_t>(a2 ^ a3)));
        col[3] ^= static_cast<uint8_t>(h ^ xtime(static_cast<uint8_t>(a3 ^ a0)));
      }
      for (size_t i = 0; i < 16; i++)
        s[i] ^= this->rk_[round * 16 + i];
    }
 
    for (uint8_t &b : s)
      b = SBOX[b];
    shift_rows(s);
    for (size_t i = 0; i < 16; i++)
      s[i] ^= this->rk_[160 + i];
    memcpy(out, s, 16);
  }
 
 protected:
  static void shift_rows(uint8_t s[16]) {
    uint8_t t = s[1];
    s[1] = s[5];
    s[5] = s[9];
    s[9] = s[13];
    s[13] = t;
    t = s[2];
    s[2] = s[10];
    s[10] = t;
    t = s[6];
    s[6] = s[14];
    s[14] = t;
    t = s[3];
    s[3] = s[15];
    s[15] = s[11];
    s[11] = s[7];
    s[7] = t;
  }
 
  uint8_t rk_[176];
};
 
}  // namespace
 
void aes128_encrypt_block(const uint8_t key[16], const uint8_t in[16], uint8_t out[16]) {
  Aes128 aes(key);
  aes.encrypt(in, out);
}
 
bool aes_ccm_auth_decrypt(const uint8_t key[16], const uint8_t *nonce, size_t nonce_len, const uint8_t *aad,
                          size_t aad_len, const uint8_t *ciphertext, size_t ct_len, uint8_t *plaintext,
                          const uint8_t *tag, size_t tag_len) {
  // CCM length field width L and tag width M (RFC 3610 §2.2). For a 13-byte
  // nonce L = 2; BTHome uses M = 4.
  if (nonce_len < 7 || nonce_len > 13 || tag_len < 4 || tag_len > 16)
    return false;
  const size_t l = 15 - nonce_len;
  const size_t m = tag_len;
 
  const Aes128 aes(key);
 
  // Build CTR block A_i = [L-1] | nonce | counter(L bytes, big-endian).
  uint8_t a[16];
  auto build_ctr = [&](uint32_t counter) {
    a[0] = static_cast<uint8_t>(l - 1);
    memcpy(a + 1, nonce, nonce_len);
    memset(a + 1 + nonce_len, 0, l);
    for (size_t i = 0; i < l; i++)
      a[15 - i] = static_cast<uint8_t>((counter >> (8 * i)) & 0xff);
  };
 
  // S_0 = E(A_0); its first m bytes mask the transmitted tag.
  uint8_t s0[16];
  build_ctr(0);
  aes.encrypt(a, s0);
 
  // CTR-decrypt ciphertext into plaintext using S_1, S_2, ...
  uint8_t ks[16];
  for (size_t off = 0; off < ct_len; off += 16) {
    build_ctr(static_cast<uint32_t>(off / 16) + 1);
    aes.encrypt(a, ks);
    const size_t n = std::min(static_cast<size_t>(16), ct_len - off);
    for (size_t i = 0; i < n; i++)
      plaintext[off + i] = static_cast<uint8_t>(ciphertext[off + i] ^ ks[i]);
  }
 
  // CBC-MAC over B_0 | (formatted AAD) | plaintext.
  uint8_t x[16];
  uint8_t b0[16];
  const uint8_t flags = static_cast<uint8_t>((aad_len > 0 ? 0x40 : 0x00) | (((m - 2) / 2) << 3) | (l - 1));
  b0[0] = flags;
  memcpy(b0 + 1, nonce, nonce_len);
  memset(b0 + 1 + nonce_len, 0, l);
  for (size_t i = 0; i < l; i++)
    b0[15 - i] = static_cast<uint8_t>((ct_len >> (8 * i)) & 0xff);
  aes.encrypt(b0, x);  // X_1 = E(B_0)
 
  if (aad_len > 0) {
    // Only the < 2^16-2^8 encoding is needed for BLE-sized AAD.
    uint8_t blk[16] = {0};
    blk[0] = static_cast<uint8_t>((aad_len >> 8) & 0xff);
    blk[1] = static_cast<uint8_t>(aad_len & 0xff);
    size_t ai = 0;
    size_t pos = 2;
    while (pos < 16 && ai < aad_len)
      blk[pos++] = aad[ai++];
    for (size_t i = 0; i < 16; i++)
      x[i] ^= blk[i];
    aes.encrypt(x, x);
    while (ai < aad_len) {
      memset(blk, 0, 16);
      const size_t n = std::min(static_cast<size_t>(16), aad_len - ai);
      memcpy(blk, aad + ai, n);
      ai += n;
      for (size_t i = 0; i < 16; i++)
        x[i] ^= blk[i];
      aes.encrypt(x, x);
    }
  }
 
  for (size_t off = 0; off < ct_len; off += 16) {
    uint8_t blk[16] = {0};
    const size_t n = std::min(static_cast<size_t>(16), ct_len - off);
    memcpy(blk, plaintext + off, n);
    for (size_t i = 0; i < 16; i++)
      x[i] ^= blk[i];
    aes.encrypt(x, x);
  }
 
  // Expected tag U = T XOR S_0[0..m). Constant-time compare with the received tag.
  uint8_t diff = 0;
  for (size_t i = 0; i < m; i++)
    diff |= static_cast<uint8_t>((x[i] ^ s0[i]) ^ tag[i]);
  return diff == 0;
}
 
}  // namespace esphome::ble_device_base