SAML Assertion in a XML using C#

try following :

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)
        {
            string subject = "<saml2:NameID Format=\"urn:oasis:names:tc:SAML:1.1:nameidformat:X509SubjectName\">UID=WilmaAnderson</saml2:NameID>" +
                              "<saml2:SubjectConfirmation Method=\"urn:oasis:names:tc:SAML:2.0:cm:holder-ofkey\">" +
                                "<saml2:SubjectConfirmationData>" +
                                  "<ds:KeyInfo>" +
                                    "<ds:KeyValue>" +
                                      "<ds:RSAKeyValue>" +
                                        "<ds:Modulus>" +
                                          "maFp5lsEHjhrQQCL0e8cbxMoDpGk1r6Uion/LA2iuD3T+sspJh8TNToQrurpFFVY/u6IzHKIk64s9894pxbwYNwv/LzRVz" +
                                          "M5pOXmCT73KGAg3Cry+QOO5KrN8hR/OXyH90/LIS77FZY+bepqD6qx4URa2/GLll08fu8xh1wPFDMCFAmb2Xz/5gK0fimUXJAWi1+PlNKML" +
                                          "nDGxHgvz5ZwiN1/QwXcQEc1mcJCimLoiCSrk7nvmqkxX4ZZ1dYzQJWdlB8Om4r9Uu96q5cZFTYwSdivLpPFKSzn/2MI9NryZC0VaIBuHRhg" +
                                          "AmspAzM90BjLO5vtiwrrfx/E3uYcMjcSEQ==" +
                                        "</ds:Modulus>" +
                                        "<ds:Exponent>AQAB</ds:Exponent>" +
                                      "</ds:RSAKeyValue>" +
                                    "</ds:KeyValue>" +
                                  "</ds:KeyInfo>" +
                                "</saml2:SubjectConfirmationData>" +
                              "</saml2:SubjectConfirmation>";

            xSubject.InnerXml = subject;
        }
    }
}