Encrypt XML so that people can't look at its data

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XML

encryption

Hello, I created a class called "ClassA" and I write its content to hard drive as an XML file like this:

static void Write(ClassA cA)
{
    var serializer = new XmlSerializer(typeof(ClassA));

    using (var stream = new FileStream(System.IO.Path.Combine("C:\\","AFile.XML"), FileMode.Create))
    {
        serializer.Serialize(stream, cA);
        stream.Close();
        stream.Dispose();
    }
}

And I read it like this:

static ClassA Load()
{
    var serializer = new XmlSerializer(typeof(ClassA));
        using (FileStream stream = new FileStream(System.IO.Path.Combine("C:\\","AFile.XML"), FileMode.Open))
        {

            var container = serializer.Deserialize(stream) as ClassA;
            stream.Close();
            stream.Dispose();
            return container;
        }
}

The problem is, the XML file is - obviously - plain text and everyone can see and change its values. How can I encrypt the whole XML file so that people can't easily manipulate it? I would be glad if you help step by step.
Thanks.

What I have tried:

I tried this for encrypting, but it kept saying "Wrong Key length" and I didn't actually get it whether it encrypts whole XML file or just the values. Overall, it just did not work and kept saying the error.

string sKey = //I tried 128, 32 and 256 bytes of string, none worked;

 var serializer = new XmlSerializer(obj.GetType());
 var stream = new FileStream(path, FileMode.Create);

 DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
 DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
 DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);

 ICryptoTransform desencrypt = DES.CreateEncryptor();

 using(CryptoStream cStream = new CryptoStream(stream, desencrypt, CryptoStreamMode.Write)){
 serializer.Serialize(cStream, obj);
 }

 stream.Close ();

The sample code is in the link below, You can find the other part (reading, decrypting) there.
Encrypting an XML File : Unity3D[^]