Method decryptToByteArray

app/src/Licensing/SimpleCrypt.cpp:261–333

* @brief Decrypts a cyphertext binary encrypted with this class with the set key */

 * @brief Decrypts a cyphertext binary encrypted with this class with the set key
 */
QByteArray Licensing::SimpleCrypt::decryptToByteArray(const QByteArray& cypher)
{
  if (m_keyParts.isEmpty()) {
    qWarning() << "No key set.";
    m_lastError = ErrorNoKeySet;
    return QByteArray();
  }

  QByteArray ba = cypher;
  if (cypher.size() < 3)
    return QByteArray();

  char version = ba.at(0);
  if (version != 3) {
    m_lastError = ErrorUnknownVersion;
    return QByteArray();
  }

  CryptoFlags flags = CryptoFlags(ba.at(1));
  ba                = ba.mid(2);
  int pos(0);
  int cnt(ba.size());
  char lastChar = 0;
  while (pos < cnt) {
    char currentChar = ba[pos];
    ba[pos]          = ba.at(pos) ^ lastChar ^ m_keyParts.at(pos % 8);
    lastChar         = currentChar;
    ++pos;
  }

  ba = ba.mid(1);

  bool integrityOk(true);
  if (flags.testFlag(CryptoFlagChecksum)) {
    if (ba.length() < 2) {
      m_lastError = ErrorIntegrityFailed;
      return QByteArray();
    }
    quint16 storedChecksum;
    {
      QDataStream s(&ba, QIODevice::ReadOnly);
      s >> storedChecksum;
    }

    ba               = ba.mid(2);
    quint16 checksum = qChecksum(ba, Qt::ChecksumIso3309);
    integrityOk      = (checksum == storedChecksum);
  }

  else if (flags.testFlag(CryptoFlagHash)) {
    if (ba.length() < 20) {
      m_lastError = ErrorIntegrityFailed;
      return QByteArray();
    }

    QByteArray storedHash = ba.left(20);
    ba                    = ba.mid(20);
    QCryptographicHash hash(QCryptographicHash::Sha1);