Not that this is going to be a regular feature or anything, but I learn something new about Gnus (my mail client) just about every day. Often what I learn is something that I knew before but forgot.
Today I re-learned that in a mail buffer,
C-c RET c pwill sign and encrypt the message.
C-u C-c RET Pruns
(mml-preview), which will show you a preview of the message after you've added the sign and encrypt tags. This way you can make sure it will actually end up being encrypted and signed.