decrypt-xml.cs

public static void Decrypt(XmlDocument Doc, SymmetricAlgorithm Alg)
{
    if (Doc == null)
        throw new ArgumentNullException("Doc");
    if (Alg == null)
        throw new ArgumentNullException("Alg");

    XmlElement encryptedElement = Doc.GetElementsByTagName("EncryptedData")[0] as XmlElement;

    if (encryptedElement == null)
    {
        throw new XmlException("The XML element was not found.");
    }

    EncryptedData edElement = new EncryptedData();
    edElement.LoadXml(encryptedElement);

    EncryptedXml exml = new EncryptedXml();

    byte[] rgbOutput = exml.DecryptData(edElement, Alg);
    exml.ReplaceData(encryptedElement, rgbOutput);
}