message: Change the computation of the start of the ciphertext pointer from
  &ciphertext[BLOCK_SIZE] to &ciphertext[0] + BLOCK_SIZE in order to avoid
  UB in case the size of the ciphertext is exactly equal to BLOCK_SIZE.
diff: @@ -208,7 +208,8 @@ util::StatusOr<std::string> AesSivBoringSsl::DecryptDeterministically(
  size_t plaintext_size = ciphertext.size() - BLOCK_SIZE;
  std::vector<uint8_t> pt(plaintext_size);
  const uint8_t *siv = reinterpret_cast<const uint8_t*>(&ciphertext[0]);
  - const uint8_t *ct = reinterpret_cast<const uint8_t*>(&ciphertext[BLOCK_SIZE]);
  + const uint8_t* ct =
  + reinterpret_cast<const uint8_t*>(&ciphertext[0]) + BLOCK_SIZE;
  CtrCrypt(siv, ct, pt.data(), plaintext_size);
  uint8_t s2v[BLOCK_SIZE];
  