Verify Digital Signature and Decrypt Email in VB.NET - 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 VB.NET.

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 VB.NET 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.

[VB.NET 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.

Note

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

Imports System.Collections
Imports System.Text
Imports System.IO
Imports EAGetMail 'imports EAGetMail namespace

Module Module1
    Private Sub ParseEmail(ByVal emlFile As String)
        Dim oMail As New Mail("TryIt")
        oMail.Load(emlFile, False)

        If oMail.IsEncrypted Then
            Try
                ' This email is encrypted, we decrypt it by user default certificate.
                ' you can also use specified certificate like this
                ' Dim oCert As New Certificate()
                ' oCert.Load("c:\test.pfx", "pfxpassword", _
                '  Certificate.CertificateKeyLocation.CRYPT_USER_KEYSET)
                ' oMail = oMail.Decrypt( oCert )
                oMail = oMail.Decrypt(Nothing)
            Catch ep As Exception
                Console.WriteLine(ep.Message)
            End Try
        End If

        If oMail.IsSigned Then
            Try
                ' This email is digital signed.
                Dim cert As EAGetMail.Certificate = oMail.VerifySignature()

                ' 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.
                Console.WriteLine("This email contains a valid digital signature.")
            Catch ep As Exception
                Console.WriteLine(ep.Message)
            End Try
        End If

        ' Parse sender
        Console.WriteLine("From: {0}", oMail.From.ToString())

        ' Parse to, recipients
        Dim addrs As MailAddress() = oMail.[To]
        For i As Integer = 0 To addrs.Length - 1
            Console.WriteLine("To: {0}", addrs(i).ToString())
        Next

        ' Parse CC
        addrs = oMail.Cc
        For i As Integer = 0 To addrs.Length - 1
            Console.WriteLine("To: {0}", addrs(i).ToString())
        Next

        ' 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
        Dim atts As Attachment() = oMail.Attachments
        For i As Integer = 0 To atts.Length - 1
            Console.WriteLine("Attachment: {0}", atts(i).Name)
        Next
    End Sub

    Sub Main()
        ' Create a folder named "inbox" under current directory
        ' to save the email retrieved.
        Dim curpath As String = Directory.GetCurrentDirectory()
        Dim mailbox As String = [String].Format("{0}\inbox", curpath)

        ' If the folder is not existed, create it.
        If Not Directory.Exists(mailbox) Then
            Directory.CreateDirectory(mailbox)
        End If

        ' Get all *.eml files in specified folder and parse it one by one.
        Dim files As String() = Directory.GetFiles(mailbox, "*.eml")
        For i As Integer = 0 To files.Length - 1
            ParseEmail(files(i))
        Next

    End Sub
End Module

Next Section

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

Appendix

Comments

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