Verify Digital Signature and Decrypt Email in C# - S/MIME

In previous section, I introduced how to parse email. In this section, I will introduce how to verify digital signature and decrypt email in C#.

Introduction

How to sign email?

Digital signature is always signed by sender certificate. The certificate used to sign email content MUST have the public/private key pair.

First of all, the user MUST get a digital certificate for personal email protection from third-party certificate authorities such as www.verisign.com.

After the certificate is installed on the machine, it can be viewed by Control Panel -> Internet Options -> Content -> Certificates -> Personal. When you view the certificate, please note there is a line “You have a private key that corresponds to this certificate” in the certificate view, that means you are able to use this certificate to sign email content. If this line doesn’t appear, that means you are unable to sign the email content by this certificate.

To sign email content, please refer to EASendMail SMTP Component.

How to encrypt email?

Encrypting email doesn’t require sender certificate but the certificate with public key for every recipient.

For example: from@adminsystem.com sends an email to rcpt@adminsystem.com with digital signature; The digital signature contains the public key certificate for from@adminsystem.com, then rcpt@adminsystem.com can send an encrypted email with this certificate back to from@adminsystem.com; Only from@adminsystem can read this email, because this email MUST be decrypted by private key of from@adminsystem.com.

Therefore, you MUST receive an digital signed email from other people (Most email clients such as outlook, outlook express will add the certificate to the Other People Storage automatically once an digital signed email is received) before you can send encrypted email to this people.

To encrypt email, please refer to EASendMail SMTP Component.

EAGetMail Mail class provides an easy way to verify the email digital signature and get the signer certificate. The signer certificate only contains the public key, that means you can add this certificate to your user certificate storage so that you can use this certificate to encrypt email and send the encrypted email back to the sender, only the sender can decrypt the email.

Note

Remarks: All of examples in this section are based on first section: A simple C# project. To compile and run the following example codes successfully, please click here to learn how to create the test project and add reference to your project.

[C# Example - Verify digital signature and decrypt email]

The following example codes demonstrate how to use EAGetMail POP3 component to verify digital signature and decrypt email. In order to run it correctly, please change email server, user, password, folder, file name values.

Note

To get the full sample projects, please refer to Samples section.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.IO;
using EAGetMail; //add EAGetMail namespace

namespace receiveemail
{
    class Program
    {
       private static void ParseEmail(string emlFile)
        {
            Mail oMail = new Mail("TryIt");
            oMail.Load(emlFile, false);

             if (oMail.IsEncrypted)
            {
                try
                {
                    // This email is encrypted, we decrypt it by user default certificate.
                    // You can also use specified certificate like this
                    // Certificate oCert = new Certificate();
                    // oCert.Load("c:\test.pfx", "pfxpassword",
                    //  Certificate.CertificateKeyLocation.CRYPT_USER_KEYSET);
                    // oMail = oMail.Decrypt( oCert );
                    oMail = oMail.Decrypt(null);
                }
                catch (Exception ep)
                {
                    Console.WriteLine(ep.Message);
                }
            }

            if (oMail.IsSigned)
            {
                try
                {
                    // This email is digital signed.
                    EAGetMail.Certificate cert = oMail.VerifySignature();
                    Console.WriteLine("This email contains a valid digital signature.");

                    // You can add the certificate to your certificate storage like this
                    // cert.AddToStore(
                    //    Certificate.CertificateStoreLocation.CERT_SYSTEM_STORE_CURRENT_USER,
                    //  "addressbook" );
                    // Then you can use send the encrypted email back to this sender.
                }
                catch (Exception ep)
                {
                    Console.WriteLine(ep.Message);
                }
            }

            // Parse Sender
            Console.WriteLine("From: {0}", oMail.From.ToString());

            // Parse To/Recipient
            MailAddress[] addrs = oMail.To;
            for (int i = 0; i < addrs.Length; i++)
            {
                Console.WriteLine("To: {0}", addrs[i].ToString());
            }

            // Parse CC
            addrs = oMail.Cc;
            for (int i = 0; i < addrs.Length; i++)
            {
                Console.WriteLine("To: {0}", addrs[i].ToString());
            }

            // Parse subject
            Console.WriteLine("Subject: {0}", oMail.Subject);

            // Parse Text/Plain body
            Console.WriteLine("TextBody: {0}", oMail.TextBody);

            // Parse HTML body
            Console.WriteLine("HtmlBody: {0}", oMail.HtmlBody );

            // Parse attachments
            Attachment[] atts = oMail.Attachments;
            for (int i = 0; i < atts.Length; i++)
            {
                Console.WriteLine("Attachment: {0}", atts[i].Name);
            }
        }

        static void Main(string[] args)
        {
            string curpath = Directory.GetCurrentDirectory();
            string mailbox = String.Format("{0}\\inbox", curpath);

            // If the folder is not existed, create it.
            if (!Directory.Exists(mailbox))
            {
                Directory.CreateDirectory(mailbox);
            }

            // Get all *.eml files in specified folder and parse it one by one.
            string[] files = Directory.GetFiles(mailbox, "*.eml");
            for (int i = 0; i < files.Length; i++)
            {
                ParseEmail(files[i]);
            }
        }
    }
}

Next Section

At next section I will introduce how to parse MAPI winmail.dat (TNEF/MAPI) attachment.

Appendix

Comments

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