Because I needed a change from QML hacking I've prepared the signing code. Signing will be an easier first step because we don't have to deal with recipient keys,
only with our own. The code in mailcrypto is very much a construction site, but successfully implements OpenPGPMIMEFormat and is unittested.

What remains to be done is:

• Figure out which signing mechanisms we need/want to support:
  • OpenPGPMIME seems to be the most sensible OpenPGP approach, the alternative being OpenPGPInline
  • OpenPGPMIME is compatible with most email clients, but not with all (MS Exchange Express is apparently an exception), in which case the Inline variant might be useful.
  • The alternative is SMIME encryption which is useful if you're running crypto on SMIME basis instead of OpenPGP, but I'm not sure we want to implement this right now.
• Cleanup the code once we have all mechanisms we want
• Key selection :
  • Using the email address we can query for existing private keys, in case of multiple that would need resolution though.
  • This yields a fingerprint, which we can use to fix a specific key
  • Using the fingerprint the system can then query for the actual key and use it.

There is currently no codepath to enable signing, this will be a matter of adding the key handling.