Function encode_control

src/stream.cpp:435–483

   */
  template<std::size_t max_payload_size>
  static inline std::string_view encode_control(session_t *session, const std::string_view &plaintext, std::array<std::uint8_t, max_payload_size> &tagged_cipher) {
    static_assert(
      max_payload_size >= sizeof(control_encrypted_t) + sizeof(crypto::cipher::tag_size),
      "max_payload_size >= sizeof(control_encrypted_t) + sizeof(crypto::cipher::tag_size)"
    );

    if (session->config.controlProtocolType != 13) {
      return plaintext;
    }

    auto seq = session->control.seq++;

    auto &iv = session->control.outgoing_iv;
    if (session->config.encryptionFlagsEnabled & SS_ENC_CONTROL_V2) {
      // We use the deterministic IV construction algorithm specified in NIST SP 800-38D
      // Section 8.2.1. The sequence number is our "invocation" field and the 'CH' in the
      // high bytes is the "fixed" field. Because each client provides their own unique
      // key, our values in the fixed field need only uniquely identify each independent
      // use of the client's key with AES-GCM in our code.
      //
      // The sequence number is 32 bits long which allows for 2^32 control stream messages
      // to be sent to each client before the IV repeats.
      iv.resize(12);
      std::copy_n((uint8_t *) &seq, sizeof(seq), std::begin(iv));
      iv[10] = 'H';  // Host originated
      iv[11] = 'C';  // Control stream
    } else {
      // Nvidia's old style encryption uses a 16-byte IV
      iv.resize(16);

      iv[0] = (std::uint8_t) seq;
    }

    auto packet = (control_encrypted_p) tagged_cipher.data();

    auto bytes = session->control.cipher.encrypt(plaintext, packet->payload(), &iv);
    if (bytes <= 0) {
      BOOST_LOG(error) << "Couldn't encrypt control data"sv;
      return {};
    }

    std::uint16_t packet_length = bytes + crypto::cipher::tag_size + sizeof(control_encrypted_t::seq);

    packet->encryptedHeaderType = util::endian::little(0x0001);
    packet->length = util::endian::little(packet_length);
    packet->seq = util::endian::little(seq);

    return std::string_view {(char *) tagged_cipher.data(), packet_length + sizeof(control_encrypted_t) - sizeof(control_encrypted_t::seq)};
  }

  int start_broadcast(broadcast_ctx_t &ctx);
  void end_broadcast(broadcast_ctx_t &ctx);