iStern Blog

Final part in the serie c# encrypting mail.

So now we got the content build and signed. Now what is left to do i encrypting the content.

In this post we have a look at how the structure of the content should just before it is encrypted, and how to encrypt the content.

Link to part 1, part 2, part 3, part 4

To encrypt the content I will use the GetCert method that we did in part 4 of this series.

As with the signing part we will split this encryption into two bits. adding boundary to the content and encrypting part.

First the encrypting part:

public byte[] DoEncrypt(string message, X509Certificate2 encryptionCertificates)
{
byte[] messageBytes = Encoding.ASCII.GetBytes(message);

EnvelopedCms envelopedCms = new EnvelopedCms(new ContentInfo(messageBytes));

CmsRecipient recipients = new CmsRecipient(SubjectIdentifierType.IssuerAndSerialNumber, encryptionCertificates);

envelopedCms.Encrypt(recipients);

return envelopedCms.Encode();
}

With this function done we can now ready the content for encryptions. We add a simple boundary to the content and specifies what type the innner boundary is of. Important that if you skipped the signing part that you change this accordingly to match the type of you content.

Here is the code for encrypting and sending the mail:

public void encrypt(string content)
{
MailMessage message = new System.Net.Mail.MailMessage();
string encryptedContentType = "application/x-pkcs7-mime; smime-type=enveloped-data; name=\"smime.p7m\"";
string signatureBoundry2 = "--PTBoundry=3";
StringBuilder fullUnencryptedMessageBuilder = new StringBuilder();
fullUnencryptedMessageBuilder.Append("Content-Type: ");
fullUnencryptedMessageBuilder.Append("multipart/signed; ");
fullUnencryptedMessageBuilder.Append(" boundary=\"");
fullUnencryptedMessageBuilder.Append(signatureBoundry2);
fullUnencryptedMessageBuilder.Append("\"; protocol=\"application/x-pkcs7-signature\"; micalg=SHA1; ");

fullUnencryptedMessageBuilder.Append("\r\n");
fullUnencryptedMessageBuilder.Append("Content-Transfer-Encoding: ");
fullUnencryptedMessageBuilder.Append(TransferEncoding.SevenBit);
fullUnencryptedMessageBuilder.Append("\r\n\r\n");
fullUnencryptedMessageBuilder.Append(content);

string fullUnencryptedMessage = fullUnencryptedMessageBuilder.ToString();

byte[] encryptedBytes = DoEncrypt(fullUnencryptedMessage, GetCert());

MemoryStream stream = new MemoryStream(encryptedBytes);
AlternateView view = new AlternateView(stream, "application/pkcs7-mime; smime-type=signed-data;name=smime.p7m");
message.AlternateViews.Add(view);
message.To.Add("youremail@yourdomain.com");

message.From = new MailAddress("someone@yourdomain.com");
message.Subject = "TEST";
SmtpClient smtp = new SmtpClient("smtp.yourdomain.com");
smtp.Send(message);

}

This function also sends out the mail you might wanna spilt this up to seperate methods. And do remember to give valide email adresses and a sane smtphost.

Now lets have a look at the result:

Yes there is a warning with the signature but rember in part 1 where did a selfsigned certificate this is the problem, but you cal always just accept the signature as trusted in your outlook.

Okay so how doesn’t the content that we build look like just before it is encrypted ?

If hightligthe the different steps we done in this series.

Also note the diffenrent boundary references from one part to underlaying content part.

Remember to change email smtp host and serial for certificat

Note you should implement all the explained methods in nice class’s that wraps all this functionality into one simple class. The code file you can download is constructed as a simple file so i would be easier to explain howto encrypt and sign emails. It is not intended to be used for production purposes.

Finished you now know ho to encrypt and sign mails with c#.

c# encrypting and signing mail