parsing signed and encrypted email - encryption

Question

I am writing an application that must be able to read signed and encrypted emails and parse through their contents. I am able to get everything working fine for emails that are only encrypted, but do not know what to do when I get an email that is also signed. Once I decrypt this email instead of having an anticipated 4 parts in a Multipart object, I have only 1 part in a MimePart object with the file name smime.p7m. I do not know how to break this file up or verify the signature. I have found the documentation on verifying a signature (http://www.mimekit.net/docs/html/Working-With-SMime.htm#Verify), but I don't see how this does anything. Obviously there is something that I am just not understanding at this point.
Below is a sample of the code that I am using. Note that this will be refactored after I get everything figured out, but this code is thus far working fine for all emails that I have tested so far that are not signed (may or may not be encrypted).
public void decryptAndSendEmails()
{
List<EmailMessage> emails = getEmails();
foreach (var email in emails)
{
var decryptedEmailMessage = new EmailMessage(service);
MimeMessage message;
using (var stream = new MemoryStream(email.MimeContent.Content, false))
{
message = MimeMessage.Load(stream);
}
var pkcs7 = message.BodyParts.OfType<ApplicationPkcs7Mime>().FirstOrDefault();
if (pkcs7 != null)
{
//If the SecureMimeType has not been set as it should, set it to EnvelopedData
if (pkcs7.SecureMimeType == SecureMimeType.Unknown)
{
var content = new MemoryStream();
pkcs7.Content.DecodeTo(content);
content.Position = 0;
pkcs7 = new ApplicationPkcs7Mime(SecureMimeType.EnvelopedData, content);
}
using (var ctx = new TemporarySecureMimeContext())
{
using (var stream = File.OpenRead(ConfigurationManager.AppSettings["certLocation"]))
{
ctx.Import(stream, ConfigurationManager.AppSettings["certPassword"]);
}
var decrypted = pkcs7.Decrypt(ctx);
var decryptedParts = new List<MimePart>();
if (decrypted is Multipart)
{
decryptedParts = breakMultiPart((Multipart)decrypted);
}
else if (decrypted is MimePart)
{
decryptedParts.Add((MimePart)decrypted);
}
else
{
throw new InvalidOperationException("Unknown Mime part found");
}
var textParts = decryptedParts.Where(r => r is TextPart);
var htmlParts = textParts.Where(x => ((TextPart)x).IsHtml);
var textBodyParts = textParts.Where(x => !((TextPart)x).IsHtml);
var attachmentParts = decryptedParts.Where(r => !(r is TextPart));
if (htmlParts.Any())
{
if (htmlParts.Count() > 1)
{
throw new InvalidOperationException("multiple html body parts.");
}
var htmlPart = (TextPart)htmlParts.First();
decryptedEmailMessage.Body = new MessageBody(BodyType.HTML, htmlPart.Text);
}
else
{
//Text body
if (textBodyParts.Count() > 1)
{
throw new InvalidOperationException("multiple text body parts.");
}
var textPart = (TextPart)textBodyParts.First();
decryptedEmailMessage.Body = new MessageBody(BodyType.Text, textPart.Text);
}
foreach (var part in attachmentParts)
{
var content = new MemoryStream();
part.Content.DecodeTo(content);
content.Position = 0;
decryptedEmailMessage.Attachments.AddFileAttachment(part.FileName, content);
if (!part.IsAttachment)
{
decryptedEmailMessage.Attachments.First(r => r.Name == part.FileName).IsInline = true;
decryptedEmailMessage.Attachments.First(r => r.Name == part.FileName).ContentId = part.ContentId;
}
}
}
////do stuff with decrypted Email
}
else
{
//The email is not encrypted
decryptedEmailMessage = email;
//do stuff with decrypted Email
}
}
}

Answer 1

I have finally figured this out using a combination of the comment from #jstedfast and the information I found in Unable to decrypt p7m using MimeKit. The following is the resulting code to fix this issue:
public void decryptAndSendEmails()
{
List<EmailMessage> emails = getEmails();
foreach (var email in emails)
{
var decryptedEmailMessage = new EmailMessage(service);
MimeMessage message;
using (var stream = new MemoryStream(email.MimeContent.Content, false))
{
message = MimeMessage.Load(stream);
}
var pkcs7 = message.BodyParts.OfType<ApplicationPkcs7Mime>().FirstOrDefault();
if (pkcs7 != null)
{
using (var ctx = new TemporarySecureMimeContext())
{
using (var stream = File.OpenRead(ConfigurationManager.AppSettings["certLocation"]))
{
ctx.Import(stream, ConfigurationManager.AppSettings["certPassword"]);
}
var decrypted = pkcs7.Decrypt(ctx);
if (decrypted != null && decrypted is MimePart && ((MimePart)decrypted).FileName == "smime.p7m")
{
//We need to verify the signature
var signedDecryptedEntity = decrypted as ApplicationPkcs7Mime;
signedDecryptedEntity.Verify(ctx, out decrypted); //the real decrypted data
}
var decryptedParts = new List<MimePart>();
if (decrypted is Multipart)
{
decryptedParts = breakMultiPart((Multipart)decrypted);
}
else if (decrypted is MimePart)
{
decryptedParts.Add((MimePart)decrypted);
}
else
{
throw new InvalidOperationException("Unknown Mime part found");
}
var textParts = decryptedParts.Where(r => r is TextPart);
var htmlParts = textParts.Where(x => ((TextPart)x).IsHtml);
var textBodyParts = textParts.Where(x => !((TextPart)x).IsHtml);
var attachmentParts = decryptedParts.Where(r => !(r is TextPart));
if (htmlParts.Any())
{
if (htmlParts.Count() > 1)
{
throw new InvalidOperationException("multiple html body parts.");
}
var htmlPart = (TextPart)htmlParts.First();
decryptedEmailMessage.Body = new MessageBody(BodyType.HTML, htmlPart.Text);
}
else
{
//Text body
if (textBodyParts.Count() > 1)
{
throw new InvalidOperationException("multiple text body parts.");
}
var textPart = (TextPart)textBodyParts.First();
decryptedEmailMessage.Body = new MessageBody(BodyType.Text, textPart.Text);
}
foreach (var part in attachmentParts)
{
var content = new MemoryStream();
part.Content.DecodeTo(content);
content.Position = 0;
decryptedEmailMessage.Attachments.AddFileAttachment(part.FileName, content);
if (!part.IsAttachment)
{
decryptedEmailMessage.Attachments.First(r => r.Name == part.FileName).IsInline = true;
decryptedEmailMessage.Attachments.First(r => r.Name == part.FileName).ContentId = part.ContentId;
}
}
}
//Do Something with email (decryptedEmailMessage)
}
else
{
//The email is not encrypted
decryptedEmailMessage = email;
//Do Something with email (decryptedEmailMessage)
}
}
}