Encrypt Email in VB.NET - S/MIME with RC2, 3DES and RSAES-OAEP

In previous section, I introduced how to send email with digital signature. In this section, I will introduce how to encrypt email with digital certificate in VB.NET.


After the recipient received your email with digital signature, the recipient can get your digital certificate public key from your digital signature. Then the recipient can encrypt an email with your public key and send it to you. Only you can decrypt this email with your private key. That is how S/MIME can protect your email content. If you don’t expose your digital certificate private key to others, none can read your email which is encrypted by your public key.

If you received an email with digital signature, your email client usually stores the public key of the sender in “Control Panel” -> “Internet Options” -> “Content” -> “Certificates” -> “Other People”.

Then you can use the following code to encrypt email and send it to your recipient.


Remarks: All of samples in this section are based on first section: Send email in 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 of EASendMail to your project.

[VB.NET - Encrypt Email (S/MIME) - Example]

The following example codes demonstrate how to encrypt email with digital certificate in VB.NET.


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

Imports System.Security.Cryptography.X509Certificates
Imports EASendMail 'Add EASendMail namespace

Module Module1

    Private Function _findCertificate(storeName As String, emailAddress As String) As X509Certificate2
        Dim cert As X509Certificate2 = Nothing
        Dim store As New X509Store(storeName, StoreLocation.CurrentUser)

        Dim certfiicates As X509Certificate2Collection = store.Certificates.Find(X509FindType.FindBySubjectName, emailAddress, True)
        If certfiicates.Count > 0 Then
            cert = certfiicates(0)
        End If


        _findCertificate = cert
    End Function

    Sub Main()

            Dim oMail As New SmtpMail("TryIt")
            ' 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 encrypted email"

            ' Set email body
            oMail.TextBody = "this is a test email with email encryption (S/MIME)"

            Dim signerCertificate As X509Certificate2 = _findCertificate("My", oMail.From.Address)
            If signerCertificate Is Nothing Then
                Throw New Exception("No signer certificate found for " + oMail.From.Address + "!")
            End If

            oMail.From.Certificate2 = signerCertificate
            ' You can also load the signer certificate from a pfx file.
            ' Dim pfxPath As String = "D:\TestCerts\signer.pfx"
            ' Dim signerCertFromPfx As X509Certificate2 = New X509Certificate2(pfxPath,
            '        "nosecret",
            ' X509KeyStorageFlags.Exportable Or X509KeyStorageFlags.UserKeySet)
            ' oMail.From.Certificate2 = signerCertFromPfx

            ' If you use it in web application,
            ' please use  X509KeyStorageFlags.Exportable Or X509KeyStorageFlags.MachineKeySet

            ' If you use it in .NET core application
            ' please use X509KeyStorageFlags.Exportable Or X509KeyStorageFlags.EphemeralKeySet

            Dim count As Integer = oMail.To.Count
            For i As Integer = 0 To count - 1
                Dim oAddress As MailAddress = oMail.To(i)
                Dim encryptCert As X509Certificate2 = _findCertificate("AddressBook", oAddress.Address)
                If encryptCert Is Nothing Then
                    encryptCert = _findCertificate("My", oAddress.Address)
                End If

                If encryptCert Is Nothing Then
                    Throw New Exception("No encryption certificate found for " + oAddress.Address + "!")
                End If

                oAddress.Certificate2 = encryptCert

                ' You can also load the encryptor certificate from a cer file Like this

                ' Dim cerPath As String = "D:\TestCerts\encryptor.cer"
                ' Dim encryptCertFromFile = New X509Certificate2(cerPath)
                ' oAddress.Certificate2 = encryptCertFromFile

            ' Your SMTP server address
            Dim oServer As New SmtpServer("smtp.emailarchitect.net")

            ' User and password for ESMTP authentication
            oServer.User = "test@emailarchitect.net"
            oServer.Password = "testpassword"

            ' Most mordern SMTP servers require SSL/TLS connection now.
            ' ConnectTryTLS means if server supports SSL/TLS, SSL/TLS will be used automatically.
            oServer.ConnectType = SmtpConnectType.ConnectTryTLS

            ' If your SMTP server uses 587 port
            ' oServer.Port = 587

            ' If your SMTP server requires SSL/TLS connection on 25/587/465 port
            ' oServer.Port = 25 ' 25 or 587 or 465
            ' oServer.ConnectType = SmtpConnectType.ConnectSSLAuto

            Console.WriteLine("start to send encrypted email ...")

            Dim oSmtp As New SmtpClient()
            oSmtp.SendMail(oServer, oMail)

            Console.WriteLine("email was sent successfully!")
        Catch ep As Exception
            Console.WriteLine("failed to send email with the following error:")
        End Try

    End Sub

End Module

If you received digital signed and encrypted email by Windows Mail(Outlook Express), it looks like this:

VB.NET sign and encrypt email

Encryption Algorithm

You can use SmtpMail.EncryptionAlgorithm property to set RC2, RC4, 3DES, AES128, AES192 or AES256 encryption algorithm. RSAES-OAEP (AES128, AES192 and AES256) is recommended.

RSA-OAEP Encryption with SHA256 HASH

If you need to use RSA-OAEP encryption with sha256 scheme based on EDIFACT rule, please have a look at this topic:

RSASSA-PSS + RSA-OAEP Encryption with SHA256 hash algorithm

Next Section

At next section I will introduce how to send email with event handler.



If you have any comments or questions about above example codes, please click here to add your comments.