Instructing MML to perform security operations on a MIME part is done using the C-c C-m s key map for signing and the C-c C-m c key map for encryption, as follows.
These commands do not immediately sign or encrypt the message, they merely insert the proper MML secure tag to instruct the MML engine to perform that operation when the message is actually sent. They may perform other operations too, such as locating and retrieving a S/MIME certificate of the person you wish to send encrypted mail to. When the mml parsing engine converts your MML into a properly encoded MIME message, the secure tag will be replaced with either a part or a multipart tag. If your message contains other mml parts, a multipart tag will be used; if no other parts are present in your message a single part tag will be used. This way, message mode will do the Right Thing (TM) with signed/encrypted multipart messages.
Since signing and especially encryption often is used when sensitive
information is sent, you may want to have some way to ensure that your
mail is actually signed or encrypted. After invoking the above
sign/encrypt commands, it is possible to preview the raw article by
using C-u C-c <RET> P (
mml-preview). Then you can
verify that your long rant about what your ex-significant other or
whomever actually did with that funny looking person at that strange
party the other night, actually will be sent encrypted.
Note! Neither PGP/MIME nor S/MIME encrypt/signs RFC822 headers. They only operate on the MIME object. Keep this in mind before sending mail with a sensitive Subject line.
By default, when encrypting a message, Gnus will use the
“signencrypt” mode, which means the message is both signed and
encrypted. If you would like to disable this for a particular
message, give the
mml-secure-message-encrypt-* command a prefix
argument, e.g., C-u C-c C-m c p.
Actually using the security commands above is not very difficult. At least not compared with making sure all involved programs talk with each other properly. Thus, we now describe what external libraries or programs are required to make things work, and some small general hints.