Bug 4152

MS Outlook creates triple-wrapped mails when encrypting using S/MIME, which
apparently leads to some parsing issues with current Claws Mail (content
visible, but headers missing in message view and also compose view of a
reply).

These messages seem to work fine in other clients (namely, Apple Mail on
OS X or iOS). Thunderbird has an aging patch pending since 12 years
(https://bugzilla.mozilla.org/show_bug.cgi?id=380624).  The UI issues
indeed need some thought: How are inner and outer signatures displayed
to the user? What to do if one of them does not check?

The wrapping of S/MIME-encrypted content within another S/MIME blob for
adding an outer signature is called triple-wrapping and was described
back in 1999 in RFC 2634 (https://tools.ietf.org/html/rfc2634), which is
referred to in the S/MIME standard (version 3.2 / RFC 5751, 
https://tools.ietf.org/html/rfc5751).  A conforming S/MIME MUA should at
least be able to correctly handle incoming triple-wrapped mails.


PS: Thinking about security implications, I realized that this does not
really prevent ciphertext tampering attacks, S/MIME needs something along
PGP's MDC. Triple-wrapping can help, though, in case the UI offers the
choice to only automatically decrypt messages that were correctly wrapped
and signed by someone in a whitelist.