[misc] [documentation] Document how mu works with encrypted mail.

[shamefulCake1]

Describe the issue

Please, consider adding a few words on how mu (not mu4e) works with encrypted mail into the manual.

I presume it uses gnupg to decrypt both s/mime and openpgp sidned messages when explcitly choosing to view them, but it is interesting to know how it works when indexing.

Does it only index the subject/header?

If it does query gnupg on decrypting, and indexes encrypted content too, it sort of becomes a semi-copy of the cleartext, which is presumably unsafe if your laptop is stolen.

Or maybe the index itself should be encrypted?

Or maybe it is worth adding a "fake" maildir with cleartexts for indexing? And make sure that it is not synchronized with the imap server?

[shamefulCake1]

If anybody finds issue, this is my results:

To make the view mode verify signatures:

  (setf mm-verify-option 'always)
  (cl-pushnew "multipart/signed" gnus-buttonized-mime-types)

This won't download the keys automatically, but, I guess, downloading automatically can be set in gpg.conf.

To manually download keys:

(defun mu4e-view-snarf-pgp-key (&optional msg)
  "Snarf the pgp key for the specified message."
  (interactive)
  (let* ((msg (or msg (mu4e-message-at-point)))
          (path (mu4e-message-field msg :path))
          (cmd (format "%s verify --verbose %s"
                 mu4e-mu-binary
                 (shell-quote-argument path)))
          (output (shell-command-to-string cmd)))
    (message "mu4e-view-snarf-pgp-key" ":msg=" msg ":path=" path ":cmd=" cmd ":output=" output)
    (let ((case-fold-search nil)
          (index 0))
      (while (string-match "finger-print[[:space:]]*: \\([A-F0-9]+\\)" output index)
        (let* ((cmd (format "%s --recv %s"
                            epg-gpg-program (match-string 1 output)))
               (output (shell-command-to-string cmd)))
          (setf index (match-end 0))
          (message output))))))

will download all keys without asking, but will not automatically trust them.

To automatically sign messages on sending:

:hook
  (mu4e-compose-mode . (lambda () (mml-secure-sign)))

The function from the manual mml-secure-message-sign-pgp for some reason does no work for me.

[djcb]

I've added a note to the mu index manpage. mu index does not decrypt, and only the metadata (various headers etc.) make it to the database.