How to Digitally Sign a SAML Assertion Embedded in a SOAP Message Using an X.509 Certificate?

The code provided focuses on the generation of a SAML assertion and subsequent signing of the assertion using an X.509 certificate. It does not include the creation of the SOAP message. The code assumes that you already have an XML message with a placeholder for the SAML assertion and that the message body contains the content you want to send.

Here is an example of how you could combine the code I provided with the code to create the SOAP message:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography.Xml;
using System.Text;
using System.Threading.Tasks;
using System.Xml;

namespace Certificate
{
    class Program
    {
        const string FILENAME = @"c:\temp\test.xml";
        static void Main(string[] args)
        {
            XmlDocument doc = new XmlDocument();
            CreateSoap(doc);
            XmlElement assertion = (XmlElement)(doc.GetElementsByTagName("saml2:Assertion")[0]);
            XmlElement security = (XmlElement)(doc.GetElementsByTagName("wsse:Security")[0]);  //added 10-20-17
            XmlElement body = (XmlElement)(doc.GetElementsByTagName("soap:Body")[0]);


            using (WebClient client = new WebClient())
            {
                byte[] xmlBytes = client.DownloadData(FILENAME);
                body.InnerXml = Encoding.UTF8.GetString(xmlBytes);
            }
            string pfxpath = @"D:\Certificate\Private-cert.pfx";
            X509Certificate2 cert = new X509Certificate2(File.ReadAllBytes(pfxpath), "123456789");


            SignXmlWithCertificate(assertion, cert);
            SignXmlWithCertificate(security, cert);   //added 10-20-17

            XmlElement subject = doc.CreateElement("Subject", "saml2");
            assertion.AppendChild(subject);

            CreateSubject(subject);

            File.WriteAllText(@"D:\Certificate\digitallysigned.xml", doc.OuterXml);
        }
        public static void CreateSoap(XmlDocument doc)
        {
            DateTime date = DateTime.Now;
            string soap = string.Format(
                "<?xml version=\"1.0\"?>" +
                "<soap:Envelope" +
                " xmlns:soap=\"http://www.w3.org/2003/05/soap-envelope\"" +
                " xmlns:wsse11=\"http://docs.oasisopen.org/wss/oasis-wss-wssecurity-secext-1.1.xsd\"" +
                " xmlns:wsse=\"http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd\"" +
                " xmlns:wsu=\"http://docs.oasis-open.org/wss/2004/01/oasis-200401-wsswssecurity-utility-1.0.xsd\"" +
                " xmlns:xs=\"http://www.w3.org/2001/XMLSchema\"" +
                " xmlns:ds=\"http://www.w3.org/2000/09/xmldsig#\"" +
                " xmlns:saml=\"urn:oasis:names:tc:SAML:1.0:assertion\"" +
                " xmlns:exc14n=\"http://www.w3.org/2001/10/xml-exc-c14n#\">" +

                           "<soap:Header>" +
                                  "<To mustUnderstand=\"true\"" +
                                     " xmlns=\"http://www.w3.org/2005/08/addressing\">https://localhost:443/Gateway/PatientDiscovery/1_0/NwHINService/NwHINPatientDiscovery" +
                                  "</To>" +
                                  "<Action mustUnderstand=\"true\"" +
                                     " xmlns=\"http://www.w3.org/2005/08/addressing\">urn:hl7-org:v3:PRPA_IN201305UV02:CrossGatewayPatientDiscovery" +
                                  "</Action>" +
                                  "<ReplyTo mustUnderstand=\"true\"" +
                                     " xmlns=\"http://www.w3.org/2005/08/addressing\">" +
                                     "<Address>http://www.w3.org/2005/08/addressing/anonymous</Address>" +
                                  "</ReplyTo>" +
                                  "<MessageID mustUnderstand=\"true\"" +
                                     " xmlns=\"http://www.w3.org/2005/08/addressing\">461433e3-4591-453b-9eb6-791c7f5ff882" +
                                  "</MessageID>" +
                                  "<wsse:Security soap:mustUnderstand=\"true\">" +
                                     "<wsu:Timestamp wsu:Id=\"_1\"" +
                                        " xmlns:ns17=\"http://docs.oasis-open.org/ws-sx/wssecureconversation/200512\"" +
                                        " xmlns:ns16=\"http://schemas.xmlsoap.org/soap/envelope/\">" +
                                        "<wsu:Created>2012-06-08T18:31:44Z</wsu:Created>" +
                                        "<wsu:Expires>2012-06-08T18:36:44Z</wsu:Expires>" +
                                     "</wsu:Timestamp>" +
                                     "<saml2:Assertion ID=\"_883e64a747a5449b83821913a2b189e6\" IssueInstant=\"{0}\" Version=\"2.0\"" +
                                        " xmlns:ds=\"http://www.w3.org/2000/09/xmldsig#\"" +
                                        " xmlns:exc14n=\"http://www.w3.org/2001/10/xml-excc14n#\"" +
                                        " xmlns:saml2=\"urn:oasis:names:tc:SAML:2.0:assertion\"" +
                                        " xmlns:xenc=\"http://www.w3.org/2001/04/xmlenc#\"" +
                                        " xmlns:xs=\"http://www.w3.org/2001/XMLSchema\">" +
                                        "<saml2:Issuer Format=\"urn:oasis:names:tc:SAML:1.1:nameid-format:X509SubjectName\">CN=SAML User,OU=SU,O=SAML User,L=Los Angeles,ST=CA,C=US" +
                                        "</saml2:Issuer>" +
                                     "</saml2:Assertion>" +
                                  "</wsse:Security>" +

                                "</soap:Header>" +
                                "<soap:Body>" +
                                "</soap:Body>" +
                             "</soap:Envelope>",
                             date.ToUniversalTime().ToString("yyyy-MM-ddThh:mm:ss.fffZ"));
            //date format
            //2015-03-09T21:12:02.279Z
            doc.LoadXml(soap);

        }
        public static void SignXmlWithCertificate(XmlElement assertion, X509Certificate2 cert)
        {
            SignedXml signedXml = new SignedXml(assertion);
            signedXml.SigningKey = cert.PrivateKey;
            Reference reference = new Reference();
            reference.Uri = "";
            reference.AddTransform(new XmlDsigEnvelopedSignatureTransform());
            signedXml.AddReference(reference);

            KeyInfo keyInfo = new KeyInfo();
            keyInfo.AddClause(new KeyInfoX509Data(cert));

            signedXml.KeyInfo = keyInfo;
            signedXml.ComputeSignature();
            XmlElement xmlsig = signedXml.GetXml();

            assertion.AppendChild(xmlsig);
        }
        public static void CreateSubject(XmlElement xSubject)

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