Signing an Xml Document using RSA-SHA256 signature method issue

Question

I am using the method below to sign Xml Documents:

public static XmlDocument SignDocument(XmlDocument doc)
    {
        string signatureCanonicalizationMethod = "http://www.w3.org/2001/10/xml-exc-c14n#";
        string signatureMethod = @"http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";
        string digestMethod = @"http://www.w3.org/2001/04/xmlenc#sha256";

        string signatureReferenceURI = "#_73e63a41-156d-4fda-a26c-8d79dcade713";

        CryptoConfig.AddAlgorithm(typeof(RSAPKCS1SHA256SignatureDescription), signatureMethod);

        var signingCertificate = GetCertificate();

        SignedXml signer = new SignedXml(doc);
        signer.SigningKey = signingCertificate.PrivateKey;
        signer.KeyInfo = new KeyInfo();
        signer.KeyInfo.AddClause(new KeyInfoX509Data(signingCertificate));

        signer.SignedInfo.CanonicalizationMethod = signatureCanonicalizationMethod;
        signer.SignedInfo.SignatureMethod = signatureMethod;

        XmlDsigEnvelopedSignatureTransform envelopeTransform = new XmlDsigEnvelopedSignatureTransform();
        XmlDsigExcC14NTransform cn14Transform = new XmlDsigExcC14NTransform();

        Reference signatureReference = new Reference();
        signatureReference.Uri = signatureReferenceURI;
        signatureReference.AddTransform(envelopeTransform);
        signatureReference.AddTransform(cn14Transform);
        signatureReference.DigestMethod = digestMethod;

        signer.AddReference(signatureReference);

        signer.ComputeSignature();
        XmlElement signatureElement = signer.GetXml();

        doc.DocumentElement.AppendChild(signer.GetXml());

        return doc;
    }


        private static X509Certificate2 GetCertificate()
    {

        X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
        store.Open(OpenFlags.ReadOnly);
        X509Certificate2 card = null;
        foreach (X509Certificate2 cert in store.Certificates)
        {
            if (!cert.HasPrivateKey) continue;

            if (cert.Thumbprint.Equals("a_certain_thumb_print", StringComparison.OrdinalIgnoreCase))
            {
                card = cert;
                break;
            }
        }
        store.Close();

        return card;
    }

An exception of type System.Security.Cryptography.CryptographicException is thrown when trying to compute the signature with the error message Invalid algorithm specified. Any ideas?

Machine: Windows Server 2008 R2

.Net Framework: 4.0.

IDE: Visual Studio 2010.

Answer 1

Thanks so much for this blog. It actually solved my problem. By the way, if certificate is loaded from file, it should be exportable: X509Certificate2 x509Key = new X509Certificate2("xxxxx.pfx", "123", X509KeyStorageFlags.Exportable);