Fossa Guard Pro 1.0 S/MIME signed support

Send S/MIME signed message

Gmail mangles S/MIME signed messages differently http://fossaguard.blogspot.com/2020/04/smime-signed-messages-support-in-gmail.html

• application/pkcs7-mime with SignedData by restricting access to the signature (since G Suite uses this format to sign messages)
• multipart/signed by rearranging MIME parts of the message converting to multipart/mixed

The following approach has been implemented to send S/MIME signed emails:

• to *@gmail.com addresses multipart/mixed format used with smime.p7m attachment which contains original S/MIME multipart/signed message with signature due to the following reasons:
  • User will be able to see message content, files without any extension
  • User will be able to view content, files of the original message with the digital signature using of Fossa Guard
• to all other addresses a standard S/MIME SignedData format due to the following reasons:
• G Suite accounts use custom domains, 
• G Suite uses SignedData format internally for S/MIME signed messages
• Gmail doesn't mangle message to external addresses

Signature status indication

When User opens S/MIME signed message in Gmail UI Fossa Gard extension tries to verify the signature 

Once S/MIME signature verified the corresponding status is indicated. 

Fossa Guard replaces the content of the message by the original read from smime.p7m attachment. 

A new button `View Original  Message` becomes available to open the email in Fossa Guard View dialog with original message content and original attachments

Signature verification 

• The attached certificate chain is not used in the email signature verification procedure until added to the list of trusted. 
• Email signature verification is performed per email Sent date.