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string AddPadding(const string& b64_string) {
  if (b64_string.size() % 4 == 3) {
    return b64_string + "=";
  } else if (b64_string.size() % 4 == 2) {
    return b64_string + "==";
  }
  return b64_string;
}

// Adapted from http://www.openssl.org/docs/man1.1.0/crypto/BIO_f_base64.html
// Takes a web safe base64 encoded string (RFC 3548) and decodes it.
// Normally, web safe base64 strings have padding '=' replaced with '.',
// but we will not pad the ciphertext. We add padding here because
// openssl has trouble with unpadded strings.
string B64Decode(const string& encoded) {
  string padded = AddPadding(encoded);
  // convert from web safe -> normal base64.
  int32 index = -1;
  while ((index = padded.find_first_of('-', index + 1)) != string::npos) {
    padded[index] = '+';
  }
  index = -1;
  while ((index = padded.find_first_of('_', index + 1)) != string::npos) {
    padded[index] = '/';
  }

  // base64 decode using openssl library.
  const int32 kOutputBufferSize = 256;
  char output[kOutputBufferSize];

  BIO* b64 = BIO_new(BIO_f_base64());
  BIO_set_flags(b64, BIO_FLAGS_BASE64_NO_NL);
  BIO* bio = BIO_new_mem_buf(const_cast<char*>(padded.data()),
                             padded.length());
  bio = BIO_push(b64, bio);
  int32 out_length = BIO_read(bio, output, kOutputBufferSize);
  BIO_free_all(bio);
  return string(output, out_length);
}

{initialization_vector (16 bytes)}{ciphertext (20-byte sections)}{integrity_signature (4 bytes)}

<byte_array <xor> HMAC(encryption_key, initialization_vector || counter_bytes)>

byte_array = SerializeAsString(ExtraTagData object)
pad = hmac(encryption_key, initialization_vector ||
      counter_bytes )  // for each 20-byte section of byte_array
ciphertext = pad <xor> byte_array // for each 20-byte section of byte_array
integrity_signature = hmac(integrity_key, byte_array ||
                      initialization_vector)  // first 4 bytes
final_message = initialization_vector || ciphertext || integrity_signature

// split up according to length rules
(initialization_vector, ciphertext, integrity_signature) = final_message

// for each 20-byte section of ciphertext
pad = hmac(encryption_key, initialization_vector || counter_bytes)

// for each 20-byte section of ciphertext
byte_array = ciphertext <xor> pad

confirmation_signature = hmac(integrity_key, byte_array ||
                         initialization_vector)
success = (confirmation_signature == integrity_signature)

bool DecryptByteArray(
    const string& ciphertext, const string& encryption_key,
    const string& integrity_key, string* cleartext) {
  // Step 1. find the length of initialization vector and clear text.
  const int cleartext_length =
     ciphertext.size() - kInitializationVectorSize - kSignatureSize;
  if (cleartext_length < 0) {
    // The length cannot be correct.
    return false;
  }

  string iv(ciphertext, 0, kInitializationVectorSize);

  // Step 2. recover clear text
  cleartext->resize(cleartext_length, '\0');
  const char* ciphertext_begin = string_as_array(ciphertext) + iv.size();
  const char* const ciphertext_end = ciphertext_begin + cleartext->size();
  string::iterator cleartext_begin = cleartext->begin();

  bool add_iv_counter_byte = true;
  while (ciphertext_begin < ciphertext_end) {
    uint32 pad_size = kHashOutputSize;
    uchar encryption_pad[kHashOutputSize];

    if (!HMAC(EVP_sha1(), string_as_array(encryption_key),
              encryption_key.length(), (uchar*)string_as_array(iv),
              iv.size(), encryption_pad, &pad_size)) {
      printf("Error: encryption HMAC failed.\n");
      return false;
    }

    for (int i = 0;
         i < kBlockSize && ciphertext_begin < ciphertext_end;
         ++i, ++cleartext_begin, ++ciphertext_begin) {
      *cleartext_begin = *ciphertext_begin ^ encryption_pad[i];
    }

    if (!add_iv_counter_byte) {
      char& last_byte = *iv.rbegin();
      ++last_byte;
      if (last_byte == '\0') {
        add_iv_counter_byte = true;
      }
    }

    if (add_iv_counter_byte) {
      add_iv_counter_byte = false;
      iv.push_back('\0');
    }
  }

message ExtraTagData {
  // advertising_id can be Apple's identifier for advertising (IDFA)
  // or Android's advertising identifier. When the advertising_id is an IDFA,
  // it is the plaintext returned by iOS's [ASIdentifierManager
  // advertisingIdentifier]. For hashed_idfa, the plaintext is the MD5 hash of
  // the IDFA.  Only one of the two fields will be available, depending on the
  // version of the SDK making the request.  Later SDKs provide unhashed values.
  optional bytes advertising_id = 1;
  optional bytes hashed_idfa = 2;
}