public static void CheckSignatureWithEncryptedGrant(string fileName, IRelDecryptor decryptor)
{
    // Create a new XML document.
    XmlDocument xmlDocument = new XmlDocument();
    XmlNamespaceManager nsManager = new XmlNamespaceManager(xmlDocument.NameTable);

    // Format using white spaces.
    xmlDocument.PreserveWhitespace = true;

    // Load the passed XML file into the document. 
    xmlDocument.Load(fileName);
    nsManager.AddNamespace("dsig", SignedXml.XmlDsigNamespaceUrl);

    // Find the "Signature" node and create a new XmlNodeList object.
    XmlNodeList nodeList = xmlDocument.SelectNodes("//dsig:Signature", nsManager);

    for (int i = 0, count = nodeList.Count; i < count; i++)
    {
        XmlDocument clone = xmlDocument.Clone() as XmlDocument;
        XmlNodeList signatures = clone.SelectNodes("//dsig:Signature", nsManager);

        // Create a new SignedXml object and pass into it the XML document clone.
        SignedXml signedXml = new SignedXml(clone);

        // Load the signature node.
        signedXml.LoadXml((XmlElement)signatures[i]);

        // Set the context for license transform
        Transform trans = ((Reference)signedXml.SignedInfo.References[0]).TransformChain[0];

        if (trans is XmlLicenseTransform)
        {

            // Decryptor is used to decrypt encryptedGrant elements.
            if (decryptor != null)
                (trans as XmlLicenseTransform).Decryptor = decryptor;
        }

        // Check the signature and display the result.
        bool result = signedXml.CheckSignature();

        if (result)
            Console.WriteLine("SUCCESS: CheckSignatureWithEncryptedGrant - issuer index #" +
                                            i.ToString());
        else
            Console.WriteLine("FAILURE: CheckSignatureWithEncryptedGrant - issuer index #" +
                                            i.ToString());
    }
}