Re: pgg*.el and passphrase caching

["Michael Teichgräber" <mt@wmipf.in-berlin.de>]

Mark Trettin <mtr-dev0@gmx.de> writes:

> I *think* the problem is, that all messages also are "encrypted to self"
> and pgg.el wants to take the first Key-ID it finds (and this is the one
> of the originator of the mail).


This looks like the same I once reported on gnus-bug@gnus.org:

| Message-ID: <87he9ww7pt.fsf@iridium.renata.de>
| Subject: pgg-decrypt-region: wrong key-ID displayed
| Date: Sat, 22 Mar 2003 01:44:30 +0100
|
| when trying to decrypt a message that has been encrypted to me and to
| the sender, the sender's key-ID is displayed when PGG is prompting for
| the passphrase of _my_ key.
|
| The reason for this is the way the local `pgg-default-user-id' is
| determined within pgg-decrypt-region in pgg.el:
|
| 	 (packet (cdr (assq 1 (with-temp-buffer
| 				(insert-buffer buf)
| 				(pgg-decode-armor-region
| 				 (point-min) (point-max))))))
| 	 (key (cdr (assq 'key-identifier packet)))
| 	 (pgg-default-user-id
| 	  (if key
| 	      (concat "0x" (pgg-truncate-key-identifier key))
| 	    pgg-default-user-id))
|
| Pgg-decode-armor-region returns per example a list:
|
|     ((18)
|      (1
|       (version . 3)
|       (key-identifier . "7F362B5EDCE28EC5")  <-- sender's key-ID
|       (public-key-algorithm . ELG-E))
|      (1
|       (version . 3)
|       (key-identifier . "DC38B8B40E9C9C4B")  <-- my key ID
|       (public-key-algorithm . ELG-E))) ,
|
| so that `(cdr (assq 1 ...' leads to a `packet' containing the sender's
| key information. PGG then prompts with `GnuPG passphrase for 0xDCE28EC5:'
| instead of `...0E9C9C4B:'.
|
| A way to change this could be first to search for a packet containing
| a key identifier that equals the (long) key identifier of the key with
| user ID `pgg-default-user-id', and then--if no matching packet could
| be found--use the sequence as it is coded at the moment.
|
| This can be a bit complicated, since the user ID `pgg-default-user-id'
| may be given in various ways, so that it would be neccessary to invoke
| something similar to `(pgg-*-lookup-key pgg-default-user-id t)' to get
| a list of long key identifiers of subkeys (`ssb') of this private key.
|
| An easy approach would be to change the prompt into just `GnuPG
| passphrase:' without showing the key identifier.
|
|
| The current implementation also has the (keyboard-wearing) side
| effect, that passphrase caching in these cases does not work, since
| the passphrase of the sender's secret key obviously cannot be in my
| cache. (The easy approach would not fix this.)


I've appended a patch I used at that time to get it working the
following way:

> Is there a way to say: "Always take one of my Key-IDs"?  And then
> decrypt the messages with the cached phrase?

The interface in PGG is extended by a function
pgg-lookup-secret-keys-avail (similar to pgg-lookup-key) that should
return a list of IDs of all your secret keys. Each backend would have
to define such a function. I only implemented one for the
GnuPG-backend: pgg-gpg-lookup-secret-keys-avail.

Then, in pgg-decrypt-region, Gnus wouldn't only extract the key ID of
the first packet of the message, but those of all key packets.

This list `msg-keys' then is intersected with the `user-keys' returned
by pgg-gpg-lookup-secret-keys-avail. The first match is used as `key'
ID (in contrast to the key ID of the first packet, as it is coded in
PGG at the moment).

Because I have switched to using gpg-agent, where Gnus' passphrase
caching won't be used, I forgot about this problem.

Perhaps the appended patch can serve as an example for a fix that
covers all PGG backends.

-- 
Michael

Index: pgg-gpg.el
===================================================================
RCS file: /usr/local/cvsroot/gnus/lisp/pgg-gpg.el,v
retrieving revision 6.18
diff -u -p -r6.18 pgg-gpg.el
--- pgg-gpg.el	8 Aug 2003 23:25:24 -0000	6.18
+++ pgg-gpg.el	20 Aug 2003 07:12:52 -0000
@@ -117,6 +117,24 @@
 				     (progn (end-of-line)(point)))
 		   ":")) 8)))))
 
+(defun pgg-gpg-lookup-secret-keys-avail ()
+  "Get a list of all key IDs from secret keyring."
+  (let ((args (list "--with-colons" "--no-greeting" "--batch" 
+		    "--list-secret-keys" "--fast-list-mode"))
+	keylist)
+    (with-temp-buffer
+      (apply #'call-process pgg-gpg-program nil t nil args)
+      (goto-char (point-min))
+      (while (re-search-forward "^\\(ssb\\|sec\\|sub\\|pub\\):"  nil t)
+	(setq keylist 
+	      (cons 
+	       (substring
+		(nth 3 (split-string 
+			(buffer-substring (- (match-end 0) 1)
+					  (progn (end-of-line)(point)))
+			":")) 8) keylist))))
+    keylist))
+
 (defun pgg-gpg-encrypt-region (start end recipients &optional sign)
   "Encrypt the current region between START and END.
 If optional argument SIGN is non-nil, do a combined sign and encrypt."
Index: pgg.el
===================================================================
RCS file: /usr/local/cvsroot/gnus/lisp/pgg.el,v
retrieving revision 6.20
diff -u -p -r6.20 pgg.el
--- pgg.el	24 Jul 2003 02:58:18 -0000	6.20
+++ pgg.el	20 Aug 2003 07:12:52 -0000
@@ -192,11 +192,26 @@ the region."
   "Decrypt the current region between START and END."
   (interactive "r")
   (let* ((buf (current-buffer))
-	 (packet (cdr (assq 1 (with-temp-buffer
+	 (packets (with-temp-buffer
 				(insert-buffer-substring buf)
-				(pgg-decode-armor-region
-				 (point-min) (point-max))))))
-	 (key (cdr (assq 'key-identifier packet)))
+		    (pgg-decode-armor-region start end)))
+	 (packet (cdr (assq 1 packets)))
+	 (key (let (found-key 
+		    msg-keys 
+		    (user-keys 
+		     (pgg-lookup-secret-keys-avail)))
+		;; extract key IDs from session key packets -> msg-keys
+		(dolist (element packets msg-keys)
+		  (if (eq (car element) 1)
+		      (let ((key (assq 'key-identifier element)))
+			(if key (setq msg-keys 
+				      (cons (pgg-truncate-key-identifier
+					     (cdr key)) msg-keys))))))
+		;; intersect key IDs of available secret keys with msg-keys
+		(dolist (key user-keys found-key)
+		  (if (member key msg-keys) 
+		      (unless found-key (setq found-key key))))
+		(if found-key found-key (cdr (assq 'key-identifier packet)))))
 	 (pgg-default-user-id 
 	  (if key
 	      (concat "0x" (pgg-truncate-key-identifier key))
@@ -341,6 +356,9 @@ within the region."
 
 (defun pgg-lookup-key (string &optional type)
   (pgg-invoke "lookup-key" (or pgg-scheme pgg-default-scheme) string type))
+
+(defun pgg-lookup-secret-keys-avail ()
+  (pgg-invoke "lookup-secret-keys-avail" (or pgg-scheme pgg-default-scheme)))
 
 (defvar pgg-insert-url-function  (function pgg-insert-url-with-w3))