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Data myData = new Data();
void Laden() {
    FileStream stream = File.Open("test.xml", FileMode.OpenOrCreate, FileAccess.ReadWrite);
    XmlSerializer serializer = new XmlSerializer(typeof(Data));
    myData = (Data)serializer.Deserialize(stream);
    stream.Close();
}
void Speichern() {
    FileStream stream = File.Open("test.xml", FileMode.OpenOrCreate, FileAccess.ReadWrite);
    XmlSerializer serializer = new XmlSerializer(typeof(Data));
    serializer.Serialize(stream, myData);
    stream.Close();
}                    

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RijndaelManaged algorithm = new RijndaelManaged();
byte[] passwordBytes = Encoding.UTF8.GetBytes("Test"); //password here
byte[] saltBytes = Encoding.UTF8.GetBytes("Text"); // salt here (another string)
PasswordDeriveBytes p = new PasswordDeriveBytes(passwordBytes, saltBytes);
// sizes are devided by 8 because [ 1 byte = 8 bits ]
algorithm.IV = p.GetBytes(algorithm.BlockSize / 8);
algorithm.Key = p.GetBytes(algorithm.KeySize / 8);


XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load("test.xml");
Encrypt(xmlDoc, "Data", algorithm);
Decrypt(xmlDoc, algorithm);                    

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CryptoStream cryptStream = new CryptoStream(memoryStream, algorithm.CreateEncryptor(Encoding.UTF8.GetBytes("Test"), Encoding.UTF8.GetBytes("Text")), CryptoStreamMode.Read);                    

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fileStream.SetLength(0);
cryptStream.CopyTo(fileStream);