text: // Copyright (c) 2019 The CounosH Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #include <crypto/chacha_poly_aead.h>
  
  #include <crypto/poly1305.h>
  #include <support/cleanse.h>
  
  #include <assert.h>
  #include <string.h>
  
  #include <cstdio>
  #include <limits>
  
  #ifndef HAVE_TIMINGSAFE_BCMP
  
  int timingsafe_bcmp(const unsigned char* b1, const unsigned char* b2, size_t n)
  {
      const unsigned char *p1 = b1, *p2 = b2;
      int ret = 0;
  
      for (; n > 0; n--)
          ret |= *p1++ ^ *p2++;
      return (ret != 0);
  }
  
  #endif // TIMINGSAFE_BCMP
  
  ChaCha20Poly1305AEAD::ChaCha20Poly1305AEAD(const unsigned char* K_1, size_t K_1_len, const unsigned char* K_2, size_t K_2_len)
  {
      assert(K_1_len == CHACHA20_POLY1305_AEAD_KEY_LEN);
      assert(K_2_len == CHACHA20_POLY1305_AEAD_KEY_LEN);
      m_chacha_main.SetKey(K_1, CHACHA20_POLY1305_AEAD_KEY_LEN);
      m_chacha_header.SetKey(K_2, CHACHA20_POLY1305_AEAD_KEY_LEN);
  
      // set the cached sequence number to uint64 max which hints for an unset cache.
      // we can't hit uint64 max since the rekey rule (which resets the sequence number) is 1GB
      m_cached_aad_seqnr = std::numeric_limits<uint64_t>::max();
  }
  
  bool ChaCha20Poly1305AEAD::Crypt(uint64_t seqnr_payload, uint64_t seqnr_aad, int aad_pos, unsigned char* dest, size_t dest_len /* length of the output buffer for sanity checks */, const unsigned char* src, size_t src_len, bool is_encrypt)
  {
      // check buffer boundaries
      if (
          // if we encrypt, make sure the source contains at least the expected AAD and the destination has at least space for the source + MAC
          (is_encrypt && (src_len < CHACHA20_POLY1305_AEAD_AAD_LEN || dest_len < src_len + POLY1305_TAGLEN)) ||
          // if we decrypt, make sure the source contains at least the expected AAD+MAC and the destination has at least space for the source - MAC
          (!is_encrypt && (src_len < CHACHA20_POLY1305_AEAD_AAD_LEN + POLY1305_TAGLEN || dest_len < src_len - POLY1305_TAGLEN))) {
          return false;
      }
  
      unsigned char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
      memset(poly_key, 0, sizeof(poly_key));
      m_chacha_main.SetIV(seqnr_payload);
  
      // block counter 0 for the poly1305 key
      // use lower 32bytes for the poly1305 key
      // (throws away 32 unused bytes (upper 32) from this ChaCha20 round)
      m_chacha_main.Seek(0);
      m_chacha_main.Crypt(poly_key, poly_key, sizeof(poly_key));
  
      // if decrypting, verify the tag prior to decryption
      if (!is_encrypt) {
          const unsigned char* tag = src + src_len - POLY1305_TAGLEN;
          poly1305_auth(expected_tag, src, src_len - POLY1305_TAGLEN, poly_key);
  
          // constant time compare the calculated MAC with the provided MAC
          if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0) {
              memory_cleanse(expected_tag, sizeof(expected_tag));
              memory_cleanse(poly_key, sizeof(poly_key));
              return false;
          }
          memory_cleanse(expected_tag, sizeof(expected_tag));
          // MAC has been successfully verified, make sure we don't covert it in decryption
          src_len -= POLY1305_TAGLEN;
      }
  
      // calculate and cache the next 64byte keystream block if requested sequence number is not yet the cache
      if (m_cached_aad_seqnr != seqnr_aad) {
          m_cached_aad_seqnr = seqnr_aad;
          m_chacha_header.SetIV(seqnr_aad);
          m_chacha_header.Seek(0);
          m_chacha_header.Keystream(m_aad_keystream_buffer, CHACHA20_ROUND_OUTPUT);
      }
      // crypt the AAD (3 bytes message length) with given position in AAD cipher instance keystream
      dest[0] = src[0] ^ m_aad_keystream_buffer[aad_pos];
      dest[1] = src[1] ^ m_aad_keystream_buffer[aad_pos + 1];
      dest[2] = src[2] ^ m_aad_keystream_buffer[aad_pos + 2];
  
      // Set the playload ChaCha instance block counter to 1 and crypt the payload
      m_chacha_main.Seek(1);
      m_chacha_main.Crypt(src + CHACHA20_POLY1305_AEAD_AAD_LEN, dest + CHACHA20_POLY1305_AEAD_AAD_LEN, src_len - CHACHA20_POLY1305_AEAD_AAD_LEN);
  
      // If encrypting, calculate and append tag
      if (is_encrypt) {
          // the poly1305 tag expands over the AAD (3 bytes length) & encrypted payload
          poly1305_auth(dest + src_len, dest, src_len, poly_key);
      }
  
      // cleanse no longer required MAC and polykey
      memory_cleanse(poly_key, sizeof(poly_key));
      return true;
  }
  
  bool ChaCha20Poly1305AEAD::GetLength(uint32_t* len24_out, uint64_t seqnr_aad, int aad_pos, const uint8_t* ciphertext)
  {
      // enforce valid aad position to avoid accessing outside of the 64byte keystream cache
      // (there is space for 21 times 3 bytes)
      assert(aad_pos >= 0 && aad_pos < CHACHA20_ROUND_OUTPUT - CHACHA20_POLY1305_AEAD_AAD_LEN);
      if (m_cached_aad_seqnr != seqnr_aad) {
          // we need to calculate the 64 keystream bytes since we reached a new aad sequence number
          m_cached_aad_seqnr = seqnr_aad;
          m_chacha_header.SetIV(seqnr_aad);                                         // use LE for the nonce
          m_chacha_header.Seek(0);                                                  // block counter 0
          m_chacha_header.Keystream(m_aad_keystream_buffer, CHACHA20_ROUND_OUTPUT); // write keystream to the cache
      }
  
      // decrypt the ciphertext length by XORing the right position of the 64byte keystream cache with the ciphertext
      *len24_out = (ciphertext[0] ^ m_aad_keystream_buffer[aad_pos + 0]) |
                   (ciphertext[1] ^ m_aad_keystream_buffer[aad_pos + 1]) << 8 |
                   (ciphertext[2] ^ m_aad_keystream_buffer[aad_pos + 2]) << 16;
  
      return true;
  }
  