ZLMediaKit/webrtc/stun_packet.cc

#define MS_CLASS "RTC::StunPacket"
// #define MS_LOG_DEV

#include "stun_packet.h"

#include <cstdio>   // std::snprintf()
#include <cstring>  // std::memcmp(), std::memcpy()

#include "utils.h"

namespace RTC {

/* Class variables. */

const uint8_t StunPacket::kMagicCookie[] = {0x21, 0x12, 0xA4, 0x42};

/* Class methods. */

StunPacket* StunPacket::Parse(const uint8_t* data, size_t len) {
  if (!StunPacket::IsStun(data, len)) return nullptr;

  /*
    The message type field is decomposed further into the following
      structure:

      0                 1
      2  3  4 5 6 7 8 9 0 1 2 3 4 5
         +--+--+-+-+-+-+-+-+-+-+-+-+-+-+
         |M |M |M|M|M|C|M|M|M|C|M|M|M|M|
         |11|10|9|8|7|1|6|5|4|0|3|2|1|0|
         +--+--+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 3: Format of STUN Message Type Field

     Here the bits in the message type field are shown as most significant
     (M11) through least significant (M0).  M11 through M0 represent a 12-
     bit encoding of the method.  C1 and C0 represent a 2-bit encoding of
     the class.
   */

  // Get type field.
  uint16_t msgType = Utils::Byte::Get2Bytes(data, 0);

  // Get length field.
  uint16_t msgLength = Utils::Byte::Get2Bytes(data, 2);

  // length field must be total size minus header's 20 bytes, and must be multiple of 4 Bytes.
  if ((static_cast<size_t>(msgLength) != len - 20) || ((msgLength & 0x03) != 0)) {
    ELOG_DEBUG(
        "length field + 20 does not match total size (or it is not multiple of 4 bytes), "
        "packet discarded");

    return nullptr;
  }

  // Get STUN method.
  uint16_t msgMethod = (msgType & 0x000f) | ((msgType & 0x00e0) >> 1) | ((msgType & 0x3E00) >> 2);

  // Get STUN class.
  uint16_t msgClass = ((data[0] & 0x01) << 1) | ((data[1] & 0x10) >> 4);

  // Create a new StunPacket (data + 8 points to the received TransactionID field).
  auto packet = new StunPacket(static_cast<Class>(msgClass), static_cast<Method>(msgMethod),
                               data + 8, data, len);

  /*
      STUN Attributes

      After the STUN header are zero or more attributes.  Each attribute
      MUST be TLV encoded, with a 16-bit type, 16-bit length, and value.
      Each STUN attribute MUST end on a 32-bit boundary.  As mentioned
      above, all fields in an attribute are transmitted most significant
      bit first.

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Type                  |            Length             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Value (variable)                ....
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   */

  // Start looking for attributes after STUN header (Byte #20).
  size_t pos{20};
  // Flags (positions) for special MESSAGE-INTEGRITY and FINGERPRINT attributes.
  bool hasMessageIntegrity{false};
  bool hasFingerprint{false};
  size_t fingerprintAttrPos;  // Will point to the beginning of the attribute.
  uint32_t fingerprint;       // Holds the value of the FINGERPRINT attribute.

  // Ensure there are at least 4 remaining bytes (attribute with 0 length).
  while (pos + 4 <= len) {
    // Get the attribute type.
    auto attrType = static_cast<Attribute>(Utils::Byte::Get2Bytes(data, pos));

    // Get the attribute length.
    uint16_t attrLength = Utils::Byte::Get2Bytes(data, pos + 2);

    // Ensure the attribute length is not greater than the remaining size.
    if ((pos + 4 + attrLength) > len) {
      ELOG_DEBUG("the attribute length exceeds the remaining size, packet discarded");

      delete packet;
      return nullptr;
    }

    // FINGERPRINT must be the last attribute.
    if (hasFingerprint) {
      ELOG_DEBUG("attribute after FINGERPRINT is not allowed, packet discarded");

      delete packet;
      return nullptr;
    }

    // After a MESSAGE-INTEGRITY attribute just FINGERPRINT is allowed.
    if (hasMessageIntegrity && attrType != Attribute::FINGERPRINT) {
      ELOG_DEBUG(
          "attribute after MESSAGE-INTEGRITY other than FINGERPRINT is not allowed, "
          "packet discarded");

      delete packet;
      return nullptr;
    }

    const uint8_t* attrValuePos = data + pos + 4;

    switch (attrType) {
      case Attribute::USERNAME: {
        packet->SetUsername(reinterpret_cast<const char*>(attrValuePos),
                            static_cast<size_t>(attrLength));

        break;
      }

      case Attribute::PRIORITY: {
        // Ensure attribute length is 4 bytes.
        if (attrLength != 4) {
          ELOG_DEBUG("attribute PRIORITY must be 4 bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        packet->SetPriority(Utils::Byte::Get4Bytes(attrValuePos, 0));

        break;
      }

      case Attribute::ICE_CONTROLLING: {
        // Ensure attribute length is 8 bytes.
        if (attrLength != 8) {
          ELOG_DEBUG("attribute ICE-CONTROLLING must be 8 bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        packet->SetIceControlling(Utils::Byte::Get8Bytes(attrValuePos, 0));

        break;
      }

      case Attribute::ICE_CONTROLLED: {
        // Ensure attribute length is 8 bytes.
        if (attrLength != 8) {
          ELOG_DEBUG("attribute ICE-CONTROLLED must be 8 bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        packet->SetIceControlled(Utils::Byte::Get8Bytes(attrValuePos, 0));

        break;
      }

      case Attribute::USE_CANDIDATE: {
        // Ensure attribute length is 0 bytes.
        if (attrLength != 0) {
          ELOG_DEBUG("attribute USE-CANDIDATE must be 0 bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        packet->SetUseCandidate();

        break;
      }

      case Attribute::MESSAGE_INTEGRITY: {
        // Ensure attribute length is 20 bytes.
        if (attrLength != 20) {
          ELOG_DEBUG("attribute MESSAGE-INTEGRITY must be 20 bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        hasMessageIntegrity = true;
        packet->SetMessageIntegrity(attrValuePos);

        break;
      }

      case Attribute::FINGERPRINT: {
        // Ensure attribute length is 4 bytes.
        if (attrLength != 4) {
          ELOG_DEBUG("attribute FINGERPRINT must be 4 bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        hasFingerprint = true;
        fingerprintAttrPos = pos;
        fingerprint = Utils::Byte::Get4Bytes(attrValuePos, 0);
        packet->SetFingerprint();

        break;
      }

      case Attribute::ERROR_CODE: {
        // Ensure attribute length >= 4bytes.
        if (attrLength < 4) {
          ELOG_DEBUG("attribute ERROR-CODE must be >= 4bytes length, packet discarded");

          delete packet;
          return nullptr;
        }

        uint8_t errorClass = Utils::Byte::Get1Byte(attrValuePos, 2);
        uint8_t errorNumber = Utils::Byte::Get1Byte(attrValuePos, 3);
        auto errorCode = static_cast<uint16_t>(errorClass * 100 + errorNumber);

        packet->SetErrorCode(errorCode);

        break;
      }

      default:;
    }

    // Set next attribute position.
    pos = static_cast<size_t>(Utils::Byte::PadTo4Bytes(static_cast<uint16_t>(pos + 4 + attrLength)));
  }

  // Ensure current position matches the total length.
  if (pos != len) {
    ELOG_DEBUG("computed packet size does not match total size, packet discarded");

    delete packet;
    return nullptr;
  }

  // If it has FINGERPRINT attribute then verify it.
  if (hasFingerprint) {
    // Compute the CRC32 of the received packet up to (but excluding) the
    // FINGERPRINT attribute and XOR it with 0x5354554e.
    uint32_t computedFingerprint = Utils::Crypto::GetCRC32(data, fingerprintAttrPos) ^ 0x5354554e;

    // Compare with the FINGERPRINT value in the packet.
    if (fingerprint != computedFingerprint) {
      ELOG_DEBUG(
          "computed FINGERPRINT value does not match the value in the packet, "
          "packet discarded");

      delete packet;
      return nullptr;
    }
  }

  return packet;
}

/* Instance methods. */

StunPacket::StunPacket(Class klass, Method method, const uint8_t* transactionId,
                       const uint8_t* data, size_t size)
    : klass(klass),
      method(method),
      transactionId(transactionId),
      data(const_cast<uint8_t*>(data)),
      size(size) {
  // MS_TRACE();
}

StunPacket::~StunPacket() {
  // MS_TRACE();
}

void StunPacket::Dump() const {
  // MS_TRACE();

  // MS_DUMP("<StunPacket>");

  std::string klass;
  switch (this->klass) {
    case Class::REQUEST:
      klass = "Request";
      break;
    case Class::INDICATION:
      klass = "Indication";
      break;
    case Class::SUCCESS_RESPONSE:
      klass = "SuccessResponse";
      break;
    case Class::ERROR_RESPONSE:
      klass = "ErrorResponse";
      break;
  }
  if (this->method == Method::BINDING) {
    // MS_DUMP("  Binding %s", klass.c_str());
  } else {
    // This prints the unknown method number. Example: TURN Allocate => 0x003.
    // MS_DUMP("  %s with unknown method %#.3x", klass.c_str(),
    // static_cast<uint16_t>(this->method));
  }
  // MS_DUMP("  size: %zu bytes", this->size);

  static char transactionId[25];

  for (int i{0}; i < 12; ++i) {
    // NOTE: n must be 3 because snprintf adds a \0 after printed chars.
    std::snprintf(transactionId + (i * 2), 3, "%.2x", this->transactionId[i]);
  }
  // MS_DUMP("  transactionId: %s", transactionId);
  if (this->errorCode != 0u)
    // MS_DUMP("  errorCode: %" PRIu16, this->errorCode);
    if (!this->username.empty())
      // MS_DUMP("  username: %s", this->username.c_str());
      if (this->priority != 0u)
        // MS_DUMP("  priority: %" PRIu32, this->priority);
        if (this->iceControlling != 0u)
          // MS_DUMP("  iceControlling: %" PRIu64, this->iceControlling);
          if (this->iceControlled != 0u)
            // MS_DUMP("  iceControlled: %" PRIu64, this->iceControlled);
            if (this->hasUseCandidate)
              // MS_DUMP("  useCandidate");
              if (this->xorMappedAddress != nullptr) {
                int family;
                uint16_t port;
                std::string ip;

                Utils::IP::GetAddressInfo(this->xorMappedAddress, family, ip, port);

                // MS_DUMP("  xorMappedAddress: %s : %" PRIu16, ip.c_str(), port);
              }
  if (this->messageIntegrity != nullptr) {
    static char messageIntegrity[41];

    for (int i{0}; i < 20; ++i) {
      std::snprintf(messageIntegrity + (i * 2), 3, "%.2x", this->messageIntegrity[i]);
    }

    // MS_DUMP("  messageIntegrity: %s", messageIntegrity);
  }
  if (this->hasFingerprint) {
  }
  // MS_DUMP("  has fingerprint");

  // MS_DUMP("</StunPacket>");
}

StunPacket::Authentication StunPacket::CheckAuthentication(const std::string& localUsername,
                                                           const std::string& localPassword) {
  // MS_TRACE();

  switch (this->klass) {
    case Class::REQUEST:
    case Class::INDICATION: {
      // Both USERNAME and MESSAGE-INTEGRITY must be present.
      if (this->messageIntegrity == nullptr || this->username.empty())
        return Authentication::BAD_REQUEST;

      // Check that USERNAME attribute begins with our local username plus ":".
      size_t localUsernameLen = localUsername.length();

      if (this->username.length() <= localUsernameLen ||
          this->username.at(localUsernameLen) != ':' ||
          (this->username.compare(0, localUsernameLen, localUsername) != 0)) {
        return Authentication::UNAUTHORIZED;
      }

      break;
    }
    // This method cannot check authentication in received responses (as we
    // are ICE-Lite and don't generate requests).
    case Class::SUCCESS_RESPONSE:
    case Class::ERROR_RESPONSE: {
      // MS_ERROR("cannot check authentication for a STUN response");

      return Authentication::BAD_REQUEST;
    }
  }

  // If there is FINGERPRINT it must be discarded for MESSAGE-INTEGRITY calculation,
  // so the header length field must be modified (and later restored).
  if (this->hasFingerprint)
    // Set the header length field: full size - header length (20) - FINGERPRINT length (8).
    Utils::Byte::Set2Bytes(this->data, 2, static_cast<uint16_t>(this->size - 20 - 8));

  // Calculate the HMAC-SHA1 of the message according to MESSAGE-INTEGRITY rules.
  const uint8_t* computedMessageIntegrity = Utils::Crypto::GetHmacShA1(
      localPassword, this->data, (this->messageIntegrity - 4) - this->data);

  Authentication result;

  // Compare the computed HMAC-SHA1 with the MESSAGE-INTEGRITY in the packet.
  if (std::memcmp(this->messageIntegrity, computedMessageIntegrity, 20) == 0)
    result = Authentication::OK;
  else
    result = Authentication::UNAUTHORIZED;

  // Restore the header length field.
  if (this->hasFingerprint)
    Utils::Byte::Set2Bytes(this->data, 2, static_cast<uint16_t>(this->size - 20));

  return result;
}

StunPacket* StunPacket::CreateSuccessResponse() {
  // MS_TRACE();

  // MS_ASSERT(
  // this->klass == Class::REQUEST,
  // "attempt to create a success response for a non Request STUN packet");

  return new StunPacket(Class::SUCCESS_RESPONSE, this->method, this->transactionId, nullptr, 0);
}

StunPacket* StunPacket::CreateErrorResponse(uint16_t errorCode) {
  // MS_TRACE();

  // MS_ASSERT(
  // this->klass == Class::REQUEST,
  // "attempt to create an error response for a non Request STUN packet");

  auto response =
      new StunPacket(Class::ERROR_RESPONSE, this->method, this->transactionId, nullptr, 0);

  response->SetErrorCode(errorCode);

  return response;
}

void StunPacket::Authenticate(const std::string& password) {
  // Just for Request, Indication and SuccessResponse messages.
  if (this->klass == Class::ERROR_RESPONSE) {
    // MS_ERROR("cannot set password for ErrorResponse messages");

    return;
  }

  this->password = password;
}

void StunPacket::Serialize(uint8_t* buffer) {
  // MS_TRACE();

  // Some useful variables.
  uint16_t usernamePaddedLen{0};
  uint16_t xorMappedAddressPaddedLen{0};
  bool addXorMappedAddress =
      ((this->xorMappedAddress != nullptr) && this->method == StunPacket::Method::BINDING &&
       this->klass == Class::SUCCESS_RESPONSE);
  bool addErrorCode = ((this->errorCode != 0u) && this->klass == Class::ERROR_RESPONSE);
  bool addMessageIntegrity = (this->klass != Class::ERROR_RESPONSE && !this->password.empty());
  bool addFingerprint{true};  // Do always.

  // Update data pointer.
  this->data = buffer;

  // First calculate the total required size for the entire packet.
  this->size = 20;  // Header.

  if (!this->username.empty()) {
    usernamePaddedLen = Utils::Byte::PadTo4Bytes(static_cast<uint16_t>(this->username.length()));
    this->size += 4 + usernamePaddedLen;
  }

  if (this->priority != 0u) this->size += 4 + 4;

  if (this->iceControlling != 0u) this->size += 4 + 8;

  if (this->iceControlled != 0u) this->size += 4 + 8;

  if (this->hasUseCandidate) this->size += 4;

  if (addXorMappedAddress) {
    switch (this->xorMappedAddress->sa_family) {
      case AF_INET: {
        xorMappedAddressPaddedLen = 8;
        this->size += 4 + 8;

        break;
      }

      case AF_INET6: {
        xorMappedAddressPaddedLen = 20;
        this->size += 4 + 20;

        break;
      }

      default: {
        // MS_ERROR("invalid inet family in XOR-MAPPED-ADDRESS attribute");

        addXorMappedAddress = false;
      }
    }
  }

  if (addErrorCode) this->size += 4 + 4;

  if (addMessageIntegrity) this->size += 4 + 20;

  if (addFingerprint) this->size += 4 + 4;

  // Merge class and method fields into type.
  uint16_t typeField = (static_cast<uint16_t>(this->method) & 0x0f80) << 2;

  typeField |= (static_cast<uint16_t>(this->method) & 0x0070) << 1;
  typeField |= (static_cast<uint16_t>(this->method) & 0x000f);
  typeField |= (static_cast<uint16_t>(this->klass) & 0x02) << 7;
  typeField |= (static_cast<uint16_t>(this->klass) & 0x01) << 4;

  // Set type field.
  Utils::Byte::Set2Bytes(buffer, 0, typeField);
  // Set length field.
  Utils::Byte::Set2Bytes(buffer, 2, static_cast<uint16_t>(this->size) - 20);
  // Set magic cookie.
  std::memcpy(buffer + 4, StunPacket::kMagicCookie, 4);
  // Set TransactionId field.
  std::memcpy(buffer + 8, this->transactionId, 12);
  // Update the transaction ID pointer.
  this->transactionId = buffer + 8;
  // Add atributes.
  size_t pos{20};

  // Add USERNAME.
  if (usernamePaddedLen != 0u) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::USERNAME));
    Utils::Byte::Set2Bytes(buffer, pos + 2, static_cast<uint16_t>(this->username.length()));
    std::memcpy(buffer + pos + 4, this->username.c_str(), this->username.length());
    pos += 4 + usernamePaddedLen;
  }

  // Add PRIORITY.
  if (this->priority != 0u) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::PRIORITY));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 4);
    Utils::Byte::Set4Bytes(buffer, pos + 4, this->priority);
    pos += 4 + 4;
  }

  // Add ICE-CONTROLLING.
  if (this->iceControlling != 0u) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::ICE_CONTROLLING));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 8);
    Utils::Byte::Set8Bytes(buffer, pos + 4, this->iceControlling);
    pos += 4 + 8;
  }

  // Add ICE-CONTROLLED.
  if (this->iceControlled != 0u) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::ICE_CONTROLLED));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 8);
    Utils::Byte::Set8Bytes(buffer, pos + 4, this->iceControlled);
    pos += 4 + 8;
  }

  // Add USE-CANDIDATE.
  if (this->hasUseCandidate) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::USE_CANDIDATE));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 0);
    pos += 4;
  }

  // Add XOR-MAPPED-ADDRESS
  if (addXorMappedAddress) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::XOR_MAPPED_ADDRESS));
    Utils::Byte::Set2Bytes(buffer, pos + 2, xorMappedAddressPaddedLen);

    uint8_t* attrValue = buffer + pos + 4;

    switch (this->xorMappedAddress->sa_family) {
      case AF_INET: {
        // Set first byte to 0.
        attrValue[0] = 0;
        // Set inet family.
        attrValue[1] = 0x01;
        // Set port and XOR it.
        std::memcpy(attrValue + 2,
                    &(reinterpret_cast<const sockaddr_in*>(this->xorMappedAddress))->sin_port, 2);
        attrValue[2] ^= StunPacket::kMagicCookie[0];
        attrValue[3] ^= StunPacket::kMagicCookie[1];
        // Set address and XOR it.
        std::memcpy(
            attrValue + 4,
            &(reinterpret_cast<const sockaddr_in*>(this->xorMappedAddress))->sin_addr.s_addr, 4);
        attrValue[4] ^= StunPacket::kMagicCookie[0];
        attrValue[5] ^= StunPacket::kMagicCookie[1];
        attrValue[6] ^= StunPacket::kMagicCookie[2];
        attrValue[7] ^= StunPacket::kMagicCookie[3];

        pos += 4 + 8;

        break;
      }

      case AF_INET6: {
        // Set first byte to 0.
        attrValue[0] = 0;
        // Set inet family.
        attrValue[1] = 0x02;
        // Set port and XOR it.
        std::memcpy(attrValue + 2,
                    &(reinterpret_cast<const sockaddr_in6*>(this->xorMappedAddress))->sin6_port, 2);
        attrValue[2] ^= StunPacket::kMagicCookie[0];
        attrValue[3] ^= StunPacket::kMagicCookie[1];
        // Set address and XOR it.
        std::memcpy(
            attrValue + 4,
            &(reinterpret_cast<const sockaddr_in6*>(this->xorMappedAddress))->sin6_addr.s6_addr,
            16);
        attrValue[4] ^= StunPacket::kMagicCookie[0];
        attrValue[5] ^= StunPacket::kMagicCookie[1];
        attrValue[6] ^= StunPacket::kMagicCookie[2];
        attrValue[7] ^= StunPacket::kMagicCookie[3];
        attrValue[8] ^= this->transactionId[0];
        attrValue[9] ^= this->transactionId[1];
        attrValue[10] ^= this->transactionId[2];
        attrValue[11] ^= this->transactionId[3];
        attrValue[12] ^= this->transactionId[4];
        attrValue[13] ^= this->transactionId[5];
        attrValue[14] ^= this->transactionId[6];
        attrValue[15] ^= this->transactionId[7];
        attrValue[16] ^= this->transactionId[8];
        attrValue[17] ^= this->transactionId[9];
        attrValue[18] ^= this->transactionId[10];
        attrValue[19] ^= this->transactionId[11];

        pos += 4 + 20;

        break;
      }
    }
  }

  // Add ERROR-CODE.
  if (addErrorCode) {
    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::ERROR_CODE));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 4);

    auto codeClass = static_cast<uint8_t>(this->errorCode / 100);
    uint8_t codeNumber = static_cast<uint8_t>(this->errorCode) - (codeClass * 100);

    Utils::Byte::Set2Bytes(buffer, pos + 4, 0);
    Utils::Byte::Set1Byte(buffer, pos + 6, codeClass);
    Utils::Byte::Set1Byte(buffer, pos + 7, codeNumber);
    pos += 4 + 4;
  }

  // Add MESSAGE-INTEGRITY.
  if (addMessageIntegrity) {
    // Ignore FINGERPRINT.
    if (addFingerprint)
      Utils::Byte::Set2Bytes(buffer, 2, static_cast<uint16_t>(this->size - 20 - 8));

    // Calculate the HMAC-SHA1 of the packet according to MESSAGE-INTEGRITY rules.
    const uint8_t* computedMessageIntegrity =
        Utils::Crypto::GetHmacShA1(this->password, buffer, pos);

    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::MESSAGE_INTEGRITY));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 20);
    std::memcpy(buffer + pos + 4, computedMessageIntegrity, 20);

    // Update the pointer.
    this->messageIntegrity = buffer + pos + 4;
    pos += 4 + 20;

    // Restore length field.
    if (addFingerprint) Utils::Byte::Set2Bytes(buffer, 2, static_cast<uint16_t>(this->size - 20));
  } else {
    // Unset the pointer (if it was set).
    this->messageIntegrity = nullptr;
  }

  // Add FINGERPRINT.
  if (addFingerprint) {
    // Compute the CRC32 of the packet up to (but excluding) the FINGERPRINT
    // attribute and XOR it with 0x5354554e.
    uint32_t computedFingerprint = Utils::Crypto::GetCRC32(buffer, pos) ^ 0x5354554e;

    Utils::Byte::Set2Bytes(buffer, pos, static_cast<uint16_t>(Attribute::FINGERPRINT));
    Utils::Byte::Set2Bytes(buffer, pos + 2, 4);
    Utils::Byte::Set4Bytes(buffer, pos + 4, computedFingerprint);
    pos += 4 + 4;

    // Set flag.
    this->hasFingerprint = true;
  } else {
    this->hasFingerprint = false;
  }

  // MS_ASSERT(pos == this->size, "pos != this->size");
}
}  // namespace RTC