Email with digital signature and embedded image

c#, digital-signature, email, embedded-resource

Solution

I do think that it is not possible to sign the entire message using `System.Net.Mail`. But this might also help you:- Send Email in C# Tutorial - SSL, HTML, Embedded Image, S/MIME

Also check this:-(Send Email with Embedded Images - ImportHtml - Example):-

using System;
using System.Collections.Generic;
using System.Text;
using EASendMail; //add EASendMail namespace

namespace mysendemail
{
    class Program
    {
        static void Main(string[] args)
        {
            SmtpMail oMail = new SmtpMail("TryIt");
            SmtpClient oSmtp = new SmtpClient();

            // Set sender email address, please change it to yours
            oMail.From = "test@emailarchitect.net";

            // Set recipient email address, please change it to yours
            oMail.To = "support@emailarchitect.net";

            // Set email subject
            oMail.Subject = "test html email with attachment";

            // Your SMTP server address
            SmtpServer oServer = new SmtpServer("smtp.emailarchitect.net");

            // User and password for ESMTP authentication, if your server doesn't require
            // User authentication, please remove the following codes.            
            oServer.User = "test@emailarchitect.net";
            oServer.Password = "testpassword";

            // If your SMTP server requires SSL connection, please add this line
            // oServer.ConnectType = SmtpConnectType.ConnectSSLAuto;

            try
            {
               // Import html body and also import linked image as embedded images.
               oMail.ImportHtml( "<html><body>test <img src=\"test.gif\"> importhtml</body></html>",
                    "c:\\my picture", //test.gif is in c:\\my picture
                    ImportHtmlBodyOptions.ImportLocalPictures | ImportHtmlBodyOptions.ImportCss );

                Console.WriteLine("start to send email with embedded image...");
                oSmtp.SendMail(oServer, oMail);
                Console.WriteLine("email was sent successfully!");
            }
            catch (Exception ep)
            {
                Console.WriteLine("failed to send email with the following error:");
                Console.WriteLine(ep.Message);
            }
        }
    }
}

Problem

I have the below working code for sending digitally signed emails. I have to insert a html based signature with a gif logo which should be pulled from the assembly resources. I have googled around and found `Convert.ToBase64String()` as possible solution but Outlook does not display the image. The 2nd approach would be `LinkedResource` with `AlternateView` to embed my image, but I actually failed to get it work with the below code. I already have an `AlternateView` to send the email with the digital signature. Is it possible to somehow add the image as well? So `mailer(to, from, from_name, relay, subject, body, cc1, cc2);`. ``` private void mailer(string toaddress, string fromaddress, string fromaddress_disp, string relays, string mailsubject, string bodytext, string ccman, string cccct) { string certname = ""; MailAddress from = new MailAddress(fromaddress, fromaddress_disp); MailAddress to = new MailAddress(toaddress); MailAddress cc_man = new MailAddress(ccman); MailAddress cc_cct = new MailAddress(cccct); MailMessage message = new MailMessage(from, to); message.CC.Add(cc_man); message.CC.Add(cc_cct); message.Subject = mailsubject; message.IsBodyHtml = true; string body = "Content-Type: text/html; charset=iso-8859-1 \r\nContent-Transfer-Encoding: 8bit\r\n\r\n" + bodytext; byte[] messageData = Encoding.ASCII.GetBytes(body); ContentInfo content = new ContentInfo(messageData); SignedCms Cms = new SignedCms(new ContentInfo(messageData)); X509Store store = new X509Store(StoreLocation.CurrentUser); store.Open(OpenFlags.ReadOnly); RSACryptoServiceProvider csp = null; X509Certificate2Collection certCollection = store.Certificates; X509Certificate2 cert = null; foreach (X509Certificate2 c in certCollection) { if ((c.Subject.Contains("myEmailAddress")) && (c.FriendlyName.Contains("CompanyEmailDigSig"))) { cert = c; break; } } if (cert != null) { csp = (RSACryptoServiceProvider)cert.PrivateKey; } else { throw new Exception("Valid certificate was not found"); } CmsSigner Signer = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, cert); Cms.ComputeSignature(Signer); byte[] SignedBytes = Cms.Encode(); MemoryStream signedStream = new MemoryStream(SignedBytes); AlternateView signedView = new AlternateView(signedStream, "application/pkcs7-mime; smime-type=signed-data; name=sig.p7m"); message.AlternateViews.Add(signedView); SmtpClient client = new SmtpClient(relays); store.Close(); try { client.Send(message); } catch (Exception ex) { //exception } } ``` EDIT: I had to re-open this thread as the previously accepted answer is not good enough.

Original source