Implement custom encryption with QR-Code signatures Leave feedback

// Define class that implements IDataEncryption interface
private class CustomXOREncryption : IDataEncryption
{
    /// <summary>
    /// Gets or sets non empty key for encryption (at least one character)
    /// </summary>
    public int Key { get; set; }
    /// <summary>
    /// Encode method to encrypt string.
    /// </summary>
    /// <param name="source">Source string to encode.</param>
    /// <returns>Returns enccrypted string</returns>
    public string Encode(string source)
    {
        return Process(source);
    }
    /// <summary>
    /// Decode method to obtain decrypted string.
    /// </summary>
    /// <param name="source">Source string to decode.</param>
    /// <returns>Returns decrypted string</returns>
    public string Decode(string source)
    {
        return Process(source);
    }
    /// <summary>
    /// Using XOR operation get encoded / decoded string
    /// </summary>
    /// <param name="source"></param>
    /// <returns></returns>
    private string Process(string source)
    {
        StringBuilder src = new StringBuilder(source);
        StringBuilder dst = new StringBuilder(src.Length);
        char chTmp;
        for (int index = 0; index < src.Length; ++index)
        {
            chTmp = src[index];
            chTmp = (char)(chTmp ^ this.Key);
            dst.Append(chTmp);
        }
        return dst.ToString();
    }
}

public class DocumentSignatureData
{
    [Format("SignID")]
    public string ID { get; set; }
    [Format("SAuth")]
    public string Author { get; set; }
    [Format("SDate", "yyyy-MM-dd")]
    public DateTime Signed { get; set; }
    [Format("SDFact", "N2")]
    public decimal DataFactor { get; set; }
    [SkipSerialization]
    public string Comments { get; set; }
}

using (Signature signature = new Signature("sample.pdf"))
{
    // create data encryption
    IDataEncryption encryption = new CustomXOREncryption();
    // create custom object
    DocumentSignatureData documentSignatureData = new DocumentSignatureData()
    {
        ID = Guid.NewGuid().ToString(),
        Author = Environment.UserName,
        Signed = DateTime.Now,
        DataFactor = 11.22M
    };
    // setup QR-Code options
    QrCodeSignOptions options = new QrCodeSignOptions()
    {
        // set custom object to serialize to QR Code
        Data = documentSignatureData,
        // QR-code type
        EncodeType = QrCodeTypes.QR,
        // specify serialization encryption
        DataEncryption = encryption,
        // locate and aligh signature
        Height = 100,
        Width = 100,
        VerticalAlignment = VerticalAlignment.Bottom,
        HorizontalAlignment = HorizontalAlignment.Right,
        Margin = new Padding() { Right = 10, Bottom = 10 }
    };
    // sign document to file
    signature.Sign("QRCodeCustomEncryption.pdf", options);
}