Use proper smime-keys entry for S/MIME signatures using OpenSSL

bug#67931

* doc/misc/emacs-mime.texi (MML Definition):
* lisp/gnus/mml.el (mml-parse-1): Add chainfile parameter to sign tags.
* lisp/gnus/mml-smime.el (mml-smime-openssl-sign-query): Include the
additional certificates from smime-keys in MML tag generation as
chainfile parameters.
(mml-smime-openssl-sign): Forward chainfile entries from the parsed tag
alist to smime-sign-buffer.
; * lisp/gnus/smime.el (smime-sign-region): Fix typo in documentation.
; (smime-sign-buffer): Improve documentation to match smime-sign-region.

doc/misc/emacs-mime.texi

@@ -787,6 +787,10 @@ Parameters for @samp{sign=smime}:
 @item keyfile
 File containing key and certificate for signer.
 
+@item chainfile
+File containing an additional certificate to be included with the
+message.
+
 @end table
 
 Parameters for @samp{encrypt=smime}:

lisp/gnus/mml-smime.el

@@ -129,11 +129,15 @@ Whether the passphrase is cached at all is controlled by
     (if func
 	(funcall func handle ctl))))
 
-(defun mml-smime-openssl-sign (_cont)
-  (when (null smime-keys)
-    (customize-variable 'smime-keys)
-    (error "No S/MIME keys configured, use customize to add your key"))
-  (smime-sign-buffer (cdar smime-keys))
+(defun mml-smime-openssl-sign (cont)
+  (smime-sign-buffer
+   ;; List with key and certificate as its car, and a list of additional
+   ;; certificates to include in its cadr for smime-sign-region
+   (list
+    (cdr (assq 'keyfile cont))
+    (mapcar #'cdr (cl-remove-if-not (apply-partially #'equal 'chainfile)
+                                    cont
+                                    :key #'car-safe))))
   (goto-char (point-min))
   (while (search-forward "\r\n" nil t)
     (replace-match "\n" t t))
@@ -167,21 +171,23 @@ Whether the passphrase is cached at all is controlled by
   (when (null smime-keys)
     (customize-variable 'smime-keys)
     (error "No S/MIME keys configured, use customize to add your key"))
-  (list 'keyfile
-	(if (= (length smime-keys) 1)
-	    (cadar smime-keys)
-	  (or (let ((from (cadr (mail-extract-address-components
-				 (or (save-excursion
-				       (save-restriction
-					 (message-narrow-to-headers)
-					 (message-fetch-field "from")))
-				     "")))))
-		(and from (smime-get-key-by-email from)))
-	      (smime-get-key-by-email
-	       (gnus-completing-read "Sign this part with what signature"
-                                     (mapcar #'car smime-keys) nil nil nil
-                                     (and (listp (car-safe smime-keys))
-                                          (caar smime-keys))))))))
+  (let ((key-with-certs
+	 (if (= (length smime-keys) 1)
+	     (cdar smime-keys)
+	   (or (let ((from (cadr (mail-extract-address-components
+				  (or (save-excursion
+				        (save-restriction
+					  (message-narrow-to-headers)
+					  (message-fetch-field "from")))
+				      "")))))
+		 (and from (smime-get-key-with-certs-by-email from)))
+	       (smime-get-key-with-certs-by-email
+	        (gnus-completing-read "Sign this part with what signature"
+                                      (mapcar #'car smime-keys) nil nil nil
+                                      (and (listp (car-safe smime-keys))
+                                           (caar smime-keys))))))))
+    (append (list 'keyfile (car key-with-certs))
+            (mapcan (apply-partially #'list 'chainfile) (cadr key-with-certs)))))
 
 (defun mml-smime-get-file-cert ()
   (ignore-errors

lisp/gnus/mml.el

@@ -233,6 +233,10 @@ part.  This is for the internal use, you should never modify the value.")
 					      (if (eq (car-safe tag) 'certfile)
 						  (cdr tag)))
 					    taginfo)))
+               (chainfiles (delq nil (mapcar (lambda (tag)
+                                               (if (eq (car-safe tag) 'chainfile)
+                                                   (cdr tag)))
+                                             taginfo)))
 	       (recipients (cdr (assq 'recipients taginfo)))
 	       (sender (cdr (assq 'sender taginfo)))
 	       (location (cdr (assq 'tag-location taginfo)))
@@ -267,6 +271,10 @@ part.  This is for the internal use, you should never modify the value.")
 			    (mapcar (lambda (certfile)
 				      (list "certfile" certfile))
 				    certfiles))
+                   ,@(apply #'append
+                            (mapcar (lambda (chainfile)
+                                      (list "chainfile" chainfile))
+                                    chainfiles))
 		   ,(if recipients "recipients")
 		   ,recipients
 		   ,(if sender "sender")

lisp/gnus/smime.el

@@ -261,7 +261,7 @@ password under `cache-key'."
 If signing fails, the buffer is not modified.  Region is assumed to
 have proper MIME tags.  KEYFILE is expected to contain a PEM encoded
 private key and certificate as its car, and a list of additional
-certificates to include in its caar.  If no additional certificates is
+certificates to include in its cadr.  If no additional certificates are
 included, KEYFILE may be the file containing the PEM encoded private
 key and certificate itself."
   (smime-new-details-buffer)
@@ -327,7 +327,10 @@ is expected to contain of a PEM encoded certificate."
 
 (defun smime-sign-buffer (&optional keyfile buffer)
   "S/MIME sign BUFFER with key in KEYFILE.
-KEYFILE should contain a PEM encoded key and certificate."
+KEYFILE is expected to contain a PEM encoded private key and certificate
+as its car, and a list of additional certificates to include in its
+cadr.  If no additional certificates are included, KEYFILE may be the
+file containing the PEM encoded private key and certificate itself."
   (interactive)
   (with-current-buffer (or buffer (current-buffer))
     (unless (smime-sign-region