SignXMLUsingCertificate.cs

/// <summary>
/// Sign the xml image document using private key certificate
/// </summary>
/// <param name="xmlDocumentBuffer">Byte[] of the xml image document</param>
/// <param name="certificate">X509Certificate2 private key certificate to sign the xml document</param>
/// <param name="signedXMLPath">Path to save the signed xml document</param>
private static void SignXMLDocument(byte[] xmlDocumentBuffer, X509Certificate2 certificate, string signedXMLPath)
{
    // Load xmlDocument data in to an XML Document
    XmlDocument xmlDocument = new XmlDocument();
    string xml = Encoding.UTF8.GetString(xmlDocumentBuffer);
    xmlDocument.LoadXml(xml);

    // Sign the XML document using the certificate private key
    using (var rsaKey = certificate.PrivateKey)
    {
        var signedXml = new SignedXml(xmlDocument);
        signedXml.SigningKey = rsaKey;

        var reference = new Reference();
        reference.Uri = "";

        var env = new XmlDsigEnvelopedSignatureTransform();
        reference.AddTransform(env);

        signedXml.AddReference(reference);

        signedXml.ComputeSignature();

        var xmlDigitalSignature = signedXml.GetXml();

        xmlDocument.DocumentElement.AppendChild(xmlDocument.ImportNode(xmlDigitalSignature, true));

        xmlDocument.Save(signedXMLPath);
    }
}