lisp/gnus/mml-smime.el: Support signing by sender.

2 files changed, 56 insertions, 16 deletions

diff --git a/lisp/gnus/ChangeLog b/lisp/gnus/ChangeLog
index c1140c526f6..40600fc20a9 100644
--- a/lisp/gnus/ChangeLog
+++ b/lisp/gnus/ChangeLog
@@ -1,3 +1,13 @@
+2013-01-07  Daiki Ueno  <ueno@gnu.org>
+
+        * mml-smime.el: Support signing by sender.
+        Requested by Uwe Brauer.
+        (mml-smime-sign-with-sender): New user option analogous
+        to mml2015-sign-with-sender.
+        (mml-smime-epg-sign): Respect mml-smime-sign-with-sender.
+        (mml-smime-epg-find-usable-secret-key): New helper function copied from
+        mml2015.el.
+
 2012-12-31  Lars Magne Ingebrigtsen  <larsi@gnus.org>
 
         * gnus-msg.el (gnus-inews-insert-gcc): Don't insert Gcc headers if Gnus
diff --git a/lisp/gnus/mml-smime.el b/lisp/gnus/mml-smime.el
index 3e769d396b0..e7f9de7980d 100644
--- a/lisp/gnus/mml-smime.el
+++ b/lisp/gnus/mml-smime.el
@@ -74,6 +74,11 @@ Whether the passphrase is cached at all is controlled by
   :group 'mime-security
   :type '(repeat (string :tag "Key ID")))
 
+(defcustom mml-smime-sign-with-sender nil
+  "If t, use message sender so find a key to sign with."
+  :group 'mime-security
+  :type 'boolean)
+
 (defun mml-smime-sign (cont)
   (let ((func (nth 1 (assq mml-smime-use mml-smime-function-alist))))
     (if func
@@ -366,6 +371,24 @@ Whether the passphrase is cached at all is controlled by
           (setq pointer (cdr pointer))))
       (setq keys (cdr keys)))))
 
+;; XXX: since gpg --list-secret-keys does not return validity of each
+;; key, `mml-smime-epg-find-usable-key' defined above is not enough for
+;; secret keys.  The function `mml-smime-epg-find-usable-secret-key'
+;; below looks at appropriate public keys to check usability.
+(defun mml-smime-epg-find-usable-secret-key (context name usage)
+  (let ((secret-keys (epg-list-keys context name t))
+        secret-key)
+    (while (and (not secret-key) secret-keys)
+      (if (mml-smime-epg-find-usable-key
+           (epg-list-keys context (epg-sub-key-fingerprint
+                                   (car (epg-key-sub-key-list
+                                         (car secret-keys)))))
+           usage)
+          (setq secret-key (car secret-keys)
+                secret-keys nil)
+        (setq secret-keys (cdr secret-keys))))
+    secret-key))
+
 (autoload 'mml-compute-boundary "mml")
 
 ;; We require mm-decode, which requires mm-bodies, which autoloads
@@ -376,29 +399,36 @@ Whether the passphrase is cached at all is controlled by
   (let* ((inhibit-redisplay t)
          (context (epg-make-context 'CMS))
          (boundary (mml-compute-boundary cont))
+         (sender (message-options-get 'message-sender))
+         (signer-names (or mml-smime-signers
+                           (if (and mml-smime-sign-with-sender sender)
+                               (list (concat "<" sender ">")))))
          signer-key
          (signers
           (or (message-options-get 'mml-smime-epg-signers)
               (message-options-set
-              'mml-smime-epg-signers
-              (if (eq mm-sign-option 'guided)
-                  (epa-select-keys context "\
+               'mml-smime-epg-signers
+               (if (eq mm-sign-option 'guided)
+                   (epa-select-keys context "\
 Select keys for signing.
 If no one is selected, default secret key is used.  "
-                                   mml-smime-signers t)
-                (if mml-smime-signers
-                    (mapcar
-                     (lambda (signer)
-                       (setq signer-key (mml-smime-epg-find-usable-key
-                                         (epg-list-keys context signer t)
-                                         'sign))
-                       (unless (or signer-key
-                                   (y-or-n-p
-                                    (format "No secret key for %s; skip it? "
+                                    signer-names
+                                    t)
+                 (if (or sender mml-smime-signers)
+                     (delq nil
+                           (mapcar
+                            (lambda (signer)
+                              (setq signer-key
+                                    (mml-smime-epg-find-usable-secret-key
+                                     context signer 'sign))
+                              (unless (or signer-key
+                                          (y-or-n-p
+                                           (format
+                                            "No secret key for %s; skip it? "
                                             signer)))
-                         (error "No secret key for %s" signer))
-                       signer-key)
-                     mml-smime-signers))))))
+                                (error "No secret key for %s" signer))
+                              signer-key)
+                            signer-names)))))))
          signature micalg)
     (epg-context-set-signers context signers)
     (if mml-smime-cache-passphrase