C# SignedXml and external data in Reference

Hello,

I am trying to use C# and SignedXml to sign (and verify) some external data like this:

        static string flXML = @"C:\Temp\Example.xml";
        static string flSignedXML = @"C:\Temp\SignedExample.xml";

        static void Main(string[] args)
        {
            try
            {
                CryptoConfig.AddAlgorithm(typeof(RSAPKCS1SHA256SignatureDescription), "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256");

                // Generate a signing key.
                RSACryptoServiceProvider Key = new RSACryptoServiceProvider();

                SignXmlFile(flXML, flSignedXML, Key);

                bool result = VerifyXmlFile(flSignedXML, Key);

                if (result)
                    Console.WriteLine("The XML signature is valid.");
                else
                    Console.WriteLine("The XML signature is not valid.");
            }
            catch (CryptographicException e)
            {
                Console.WriteLine(e.Message);
            }
        }

        public static void SignXmlFile(string FileName, string SignedFileName, RSA Key)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(new XmlTextReader(FileName));

            SignedXml signedXml = new SignedXml(doc);
            signedXml.SigningKey = Key;
            signedXml.SignedInfo.SignatureMethod = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";

            // Create a reference to be signed.
            Reference reference = new Reference();
            reference.Uri = "";
            reference.AddTransform(new XmlDsigEnvelopedSignatureTransform());
            reference.AddTransform(new XmlDsigExcC14NTransform());
            reference.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256";
            signedXml.AddReference(reference);

            // Add the extgernal data to be signed to the SignedXml object.
            {
                byte[] Content = System.Text.Encoding.UTF8.GetBytes("12345qwert67890asdfg");
                Stream stream = new MemoryStream(Content);
                Reference reference2 = new Reference(stream);
                reference2.Uri = "cid:xml-sample";
                reference2.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256";
                signedXml.AddReference(reference2);
            }

            signedXml.ComputeSignature();
            XmlElement xmlDigitalSignature = signedXml.GetXml();
            doc.DocumentElement.AppendChild(doc.ImportNode(xmlDigitalSignature, true));
            if (doc.FirstChild is XmlDeclaration)
            {
                doc.RemoveChild(doc.FirstChild);
            }

            XmlTextWriter xmltw = new XmlTextWriter(SignedFileName, new UTF8Encoding(false));
            doc.WriteTo(xmltw);
            xmltw.Close();
        }

        public static Boolean VerifyXmlFile(String Name, RSA Key)
        {
            XmlDocument xmlDocument = new XmlDocument();
            xmlDocument.Load(Name);

            SignedXml signedXml = new SignedXml(xmlDocument);
            XmlNodeList nodeList = xmlDocument.GetElementsByTagName("Signature");
            signedXml.LoadXml((XmlElement)nodeList[0]);

            {
                byte[] Content = System.Text.Encoding.UTF8.GetBytes("12345qwert67890asdfg");
                Stream stream = new MemoryStream(Content);

                Reference reference2 = new Reference(stream);
                reference2.Uri = "cid:xml-sample";
                reference2.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256";
                signedXml.AddReference(reference2);
            }

            signedXml.SigningKey = Key;
            // Check the signature and return the result.
            return signedXml.CheckSignature();
        }

and CheckSignature always returns false!

Input XML:

<MyElement xmlns="samples"><PayloadInfo><PartInfo href="cid:xml-sample">                        </PartInfo></PayloadInfo></MyElement>

Signed XML looks OK to me:

<MyElement xmlns="samples"><PayloadInfo><PartInfo href="cid:xml-sample"></PartInfo></PayloadInfo><Signature xmlns="http://www.w3.org/2000/09/xmldsig#"><SignedInfo><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" /><SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" /><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" /><Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" /></Transforms><DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" /><DigestValue>pfmPOwgPEutZr/DP1U3KoBO/7dHnq0PlJm+f0Cx0HLE=</DigestValue></Reference><Reference URI="cid:xml-sample"><DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" /><DigestValue>2QmqZagw72ea4w4UXVhSt5I0AJ7lcUx5rRsvQMVnz4g=</DigestValue></Reference></SignedInfo><SignatureValue>Y/cHki0vVO2CNLHdSaGsTRVueSH/OsYmCspLByKnlcK8XM7kjcFiSuocxz8xsUVv3bk8LtgRMtDK3eZQr8WmKSBDkgEWmVF88xAArDPm0DJVZn89RX6rAIvcitnDXG/oAOwTxyW9wQoKzWqg3fyrCq4o0g2J0zr8vcPmQVrgdJg=</SignatureValue></Signature></MyElement>

Beautified:

<MyElement xmlns="samples"><PayloadInfo><PartInfo href="cid:xml-sample"></PartInfo></PayloadInfo><Signature xmlns="http://www.w3.org/2000/09/xmldsig#"><SignedInfo><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" /><SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" /><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" /><Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" /></Transforms><DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" /><DigestValue>pfmPOwgPEutZr/DP1U3KoBO/7dHnq0PlJm+f0Cx0HLE=</DigestValue></Reference><Reference URI="cid:xml-sample"><DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" /><DigestValue>2QmqZagw72ea4w4UXVhSt5I0AJ7lcUx5rRsvQMVnz4g=</DigestValue></Reference></SignedInfo><SignatureValue>Y/cHki0vVO2CNLHdSaGsTRVueSH/OsYmCspLByKnlcK8XM7kjcFiSuocxz8xsUVv3bk8LtgRMtDK3eZQr8WmKSBDkgEWmVF88xAArDPm0DJVZn89RX6rAIvcitnDXG/oAOwTxyW9wQoKzWqg3fyrCq4o0g2J0zr8vcPmQVrgdJg=</SignatureValue></Signature></MyElement>

If I do not include second Reference (external data) CheckSignature will return true.

Does anyone knows what I am doing wrong ?

Thanks,

Sasa Kajic