Encrypting and signing Mail in .Net part 3/5 (Building the content - with attachments)

Topic: How to encrypt and sign mail c#. Signed mail in c#, signing c#, c# send encrypted mail, signed encrypted mail c#, S/MIME messages

Lets get startet with the real stuff on how to encrypt mail with c#

Now we have created the certificats and we have the methods for retrieving them. So before we can sign and encrypt the mail we need to prepare there content for security operations. When sending encrypted mail with attachment the content should be included as a part of the content. If add an attahcment to an email with  using the normal "System.net.mail - message.attachments.add() " breaks the encryption. To overcome this the attachments are added to the maincontent of message but seprated with boundries.

Link to part 1, part 2

Since there are different ways to fetch filecontent and bodycontent i've have focused only on howto message data should be build, so there are left some work for you to do. Implementing functions for retrieving filecontent into byte[] fx.

To make the next couple of step easier to connecto to this one which is the backbone in encrypting the mail, I will make a simple builcontent() function that builds a simple text/plain mail and adds an attachment called snebar.jpg placed on the root of my c drive.

  

public string buildMessageContent()

{

  string messageBoundry = "--PTBoundry=2";

  StringBuilder message = new StringBuilder();

  message.Append("\r\n");

  message.Append("\r\n");

  message.Append("--");

  message.Append(messageBoundry + "\r\n");

  message.Append("Content-Type: text/plain; charset=us-ascii\r\n"); 

  //could use text/html as well here if you want a html message         

  message.Append("Content-Transfer-Encoding: ");

  message.Append(TransferEncoding.QuotedPrintable);         

  message.Append("\r\n\r\n");

  message.Append("TEST AF kryptering")//BODY TEXT GOES HERE



       

  message.Append("\r\n");



  //ADD file section

  //could be filename or whatever

  //foreach (string filename in attachments){

  //Read file part implement your own

   byte[] buff = null;

   FileStream fs = new

       FileStream("c:\\snebaer.jpg", FileMode.Open, FileAccess.Read);        

   BinaryReader br = new BinaryReader(fs);

   long numBytes = new FileInfo("c:\\snebaer.jpg").Length;       

   buff = br.ReadBytes((int)numBytes);

   byte[] bytes = buff;

   //Setup filecontent

   String filecontent =

      Convert.ToBase64String(bytes,Base64FormattingOptions.InsertLineBreaks);



  message.Append("--");

  message.Append(messageBoundry);

  message.Append("\r\n");

  message.Append("Content-Type: ");

  message.Append("application/octet-stream;");

  message.Append("name=c:\\snebaer.jpg");

  message.Append("\r\n");

  message.Append("Content-Transfer-Encoding: base64\r\n\r\n");

  message.Append(filecontent);

  message.Append("\r\n\r\n");

  //} //END FILSECTION



  message.Append("--");

  message.Append(messageBoundry);

  message.Append("--\r\n");

  return message.ToString();

}

Note that there isout comment foreach loop which could added if you need to add multiple attachments. I will also be a good idee to ad at method for building unique boundaries that could be used you can use Guid or what ever you like. I use theese static one so it easier to refence them in the next post. the string returned here is now readyto be signed. Look