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.
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) store.Open(OpenFlags.ReadOnly) Dim certfiicates As X509Certificate2Collection = store.Certificates.Find(X509FindType.FindBySubjectName, emailAddress, True) If certfiicates.Count > 0 Then cert = certfiicates(0) End If store.Close() _findCertificate = cert End Function Sub Main() Try Dim oMail As New SmtpMail("TryIt") ' Set sender email address, please change it to yours oMail.From = "email@example.com" ' Set recipient email address, please change it to yours oMail.To = "firstname.lastname@example.org" ' 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 Next ' Your SMTP server address Dim oServer As New SmtpServer("smtp.emailarchitect.net") ' User and password for ESMTP authentication oServer.User = "email@example.com" 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:") Console.WriteLine(ep.Message) End Try End Sub End Module
If you received digital signed and encrypted email by Windows Mail(Outlook Express), it looks like this:
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:
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.