Gnus can digitally sign and encrypt your messages, using vanilla
PGP format or PGP/MIME or S/MIME. For
decoding such messages, see the
mm-decrypt-option options (see Security).
Often, you would like to sign replies to people who send you signed
messages. Even more often, you might want to encrypt messages which
are in reply to encrypted messages. Gnus offers
gnus-message-replysign to enable the former, and
gnus-message-replyencrypt for the latter. In addition, setting
gnus-message-replysignencrypted (on by default) will sign
automatically encrypted messages.
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.
See Security, for more information.