1;;; canlock.el --- functions for Cancel-Lock feature
2
3;; Copyright (C) 1998, 1999, 2001, 2002, 2003, 2004,
4;;   2005, 2006, 2007 Free Software Foundation, Inc.
5
6;; Author: Katsumi Yamaoka <yamaoka@jpl.org>
7;; Keywords: news, cancel-lock, hmac, sha1, rfc2104
8
9;; This program is free software; you can redistribute it and/or modify
10;; it under the terms of the GNU General Public License as published by
11;; the Free Software Foundation; either version 2, or (at your option)
12;; any later version.
13
14;; This program is distributed in the hope that it will be useful,
15;; but WITHOUT ANY WARRANTY; without even the implied warranty of
16;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
17;; GNU General Public License for more details.
18
19;; You should have received a copy of the GNU General Public License
20;; along with this program; see the file COPYING.  If not, write to the
21;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
22;; Boston, MA 02110-1301, USA.
23
24;;; Commentary:
25
26;; Canlock is a library for generating and verifying Cancel-Lock and/or
27;; Cancel-Key header in news articles.  This is used to protect articles
28;; from rogue cancel, supersede or replace attacks.  The method is based
29;; on draft-ietf-usefor-cancel-lock-01.txt which was released on November
30;; 3rd 1998.  For instance, you can add Cancel-Lock (and possibly Cancel-
31;; Key) header in a news article by using a hook which will be evaluated
32;; just before sending an article as follows:
33;;
34;; (add-hook '*e**a*e-header-hook 'canlock-insert-header t)
35;;
36;; Verifying Cancel-Lock is mainly a function of news servers, however,
37;; you can verify your own article using the command `canlock-verify' in
38;; the (raw) article buffer.  You will be prompted for the password for
39;; each time if the option `canlock-password' or `canlock-password-for-
40;; verify' is nil.  Note that setting these options is a bit unsafe.
41
42;;; Code:
43
44(eval-when-compile
45  (require 'cl))
46
47(require 'sha1)
48
49(defvar mail-header-separator)
50
51(defgroup canlock nil
52  "The Cancel-Lock feature."
53  :group 'news)
54
55(defcustom canlock-password nil
56  "Password to use when signing a Cancel-Lock or a Cancel-Key header."
57  :type '(radio (const :format "Not specified " nil)
58		(string :tag "Password"))
59  :group 'canlock)
60
61(defcustom canlock-password-for-verify canlock-password
62  "Password to use when verifying a Cancel-Lock or a Cancel-Key header."
63  :type '(radio (const :format "Not specified " nil)
64		(string :tag "Password"))
65  :group 'canlock)
66
67(defcustom canlock-force-insert-header nil
68  "If non-nil, insert a Cancel-Lock or a Cancel-Key header even if the
69buffer does not look like a news message."
70  :type 'boolean
71  :group 'canlock)
72
73(eval-when-compile
74  (defmacro canlock-string-as-unibyte (string)
75    "Return a unibyte string with the same individual bytes as STRING."
76    (if (fboundp 'string-as-unibyte)
77	(list 'string-as-unibyte string)
78      string)))
79
80(defun canlock-sha1 (message)
81  "Make a SHA-1 digest of MESSAGE as a unibyte string of length 20 bytes."
82  (let (sha1-maximum-internal-length)
83    (sha1 message nil nil 'binary)))
84
85(defun canlock-make-cancel-key (message-id password)
86  "Make a Cancel-Key header."
87  (when (> (length password) 20)
88    (setq password (canlock-sha1 password)))
89  (setq password (concat password (make-string (- 64 (length password)) 0)))
90  (let ((ipad (mapconcat (lambda (byte)
91			   (char-to-string (logxor 54 byte)))
92			 password ""))
93	(opad (mapconcat (lambda (byte)
94			   (char-to-string (logxor 92 byte)))
95			 password "")))
96    (base64-encode-string
97     (canlock-sha1
98      (concat opad
99	      (canlock-sha1
100	       (concat ipad (canlock-string-as-unibyte message-id))))))))
101
102(defun canlock-narrow-to-header ()
103  "Narrow the buffer to the head of the message."
104  (let (case-fold-search)
105    (narrow-to-region
106     (goto-char (point-min))
107     (goto-char (if (re-search-forward
108		     (format "^$\\|^%s$"
109			     (regexp-quote mail-header-separator))
110		     nil t)
111		    (match-beginning 0)
112		  (point-max))))))
113
114(defun canlock-delete-headers ()
115  "Delete Cancel-Key or Cancel-Lock headers in the narrowed buffer."
116  (let ((case-fold-search t))
117    (goto-char (point-min))
118    (while (re-search-forward "^Cancel-\\(Key\\|Lock\\):" nil t)
119      (delete-region (match-beginning 0)
120		     (if (re-search-forward "^[^\t ]" nil t)
121			 (goto-char (match-beginning 0))
122		       (point-max))))))
123
124(defun canlock-fetch-fields (&optional key)
125  "Return a list of the values of Cancel-Lock header.
126If KEY is non-nil, look for a Cancel-Key header instead.  The buffer
127is expected to be narrowed to just the headers of the message."
128  (let ((field (mail-fetch-field (if key "Cancel-Key" "Cancel-Lock")))
129	fields rest
130	(case-fold-search t))
131    (when field
132      (setq fields (split-string field "[\t\n\r ,]+"))
133      (while fields
134	(when (string-match "^sha1:" (setq field (pop fields)))
135	  (push (substring field 5) rest)))
136      (nreverse rest))))
137
138(defun canlock-fetch-id-for-key ()
139  "Return a Message-ID in Cancel, Supersedes or Replaces header.
140The buffer is expected to be narrowed to just the headers of the
141message."
142  (or (let ((cancel (mail-fetch-field "Control")))
143	(and cancel
144	     (string-match "^cancel[\t ]+\\(<[^\t\n @<>]+@[^\t\n @<>]+>\\)"
145			   cancel)
146	     (match-string 1 cancel)))
147      (mail-fetch-field "Supersedes")
148      (mail-fetch-field "Replaces")))
149
150;;;###autoload
151(defun canlock-insert-header (&optional id-for-key id-for-lock password)
152  "Insert a Cancel-Key and/or a Cancel-Lock header if possible."
153  (let (news control key-for-key key-for-lock)
154    (save-excursion
155      (save-restriction
156	(canlock-narrow-to-header)
157	(when (setq news (or canlock-force-insert-header
158			     (mail-fetch-field "Newsgroups")))
159	  (unless id-for-key
160	    (setq id-for-key (canlock-fetch-id-for-key)))
161	  (if (and (setq control (mail-fetch-field "Control"))
162		   (string-match "^cancel[\t ]+<[^\t\n @<>]+@[^\t\n @<>]+>"
163				 control))
164	      (setq id-for-lock nil)
165	    (unless id-for-lock
166	      (setq id-for-lock (mail-fetch-field "Message-ID"))))
167	  (canlock-delete-headers)
168	  (goto-char (point-max))))
169      (when news
170	(if (not (or id-for-key id-for-lock))
171	    (message "There are no Message-ID(s)")
172	  (unless password
173	    (setq password (or canlock-password
174			       (read-passwd
175				"Password for Canlock: "))))
176	  (if (or (not (stringp password)) (zerop (length password)))
177	      (message "Password for Canlock is bad")
178	    (setq key-for-key (when id-for-key
179				(canlock-make-cancel-key
180				 id-for-key password))
181		  key-for-lock (when id-for-lock
182				 (canlock-make-cancel-key
183				  id-for-lock password)))
184	    (if (not (or key-for-key key-for-lock))
185		(message "Couldn't insert Canlock header")
186	      (when key-for-key
187		(insert "Cancel-Key: sha1:" key-for-key "\n"))
188	      (when key-for-lock
189		(insert "Cancel-Lock: sha1:"
190			(base64-encode-string (canlock-sha1 key-for-lock))
191			"\n")))))))))
192
193;;;###autoload
194(defun canlock-verify (&optional buffer)
195  "Verify Cancel-Lock or Cancel-Key in BUFFER.
196If BUFFER is nil, the current buffer is assumed.  Signal an error if
197it fails."
198  (interactive)
199  (let (keys locks errmsg id-for-key id-for-lock password
200	     key-for-key key-for-lock match)
201    (save-excursion
202      (when buffer
203	(set-buffer buffer))
204      (save-restriction
205	(widen)
206	(canlock-narrow-to-header)
207	(setq keys (canlock-fetch-fields 'key)
208	      locks (canlock-fetch-fields))
209	(if (not (or keys locks))
210	    (setq errmsg
211		  "There are neither Cancel-Lock nor Cancel-Key headers")
212	  (setq id-for-key (canlock-fetch-id-for-key)
213		id-for-lock (mail-fetch-field "Message-ID"))
214	  (or id-for-key id-for-lock
215	      (setq errmsg "There are no Message-ID(s)")))))
216    (if errmsg
217	(error "%s" errmsg)
218      (setq password (or canlock-password-for-verify
219			 (read-passwd "Password for Canlock: ")))
220      (if (or (not (stringp password)) (zerop (length password)))
221	  (error "Password for Canlock is bad")
222	(when keys
223	  (when id-for-key
224	    (setq key-for-key (canlock-make-cancel-key id-for-key password))
225	    (while (and keys (not match))
226	      (setq match (string-equal key-for-key (pop keys)))))
227	  (setq keys (if match "good" "bad")))
228	(setq match nil)
229	(when locks
230	  (when id-for-lock
231	    (setq key-for-lock
232		  (base64-encode-string
233		   (canlock-sha1 (canlock-make-cancel-key id-for-lock
234							  password))))
235	    (when (and locks (not match))
236	      (setq match (string-equal key-for-lock (pop locks)))))
237	  (setq locks (if match "good" "bad")))
238	(prog1
239	    (when (member "bad" (list keys locks))
240	      "bad")
241	  (cond ((and keys locks)
242		 (message "Cancel-Key is %s, Cancel-Lock is %s" keys locks))
243		(locks
244		 (message "Cancel-Lock is %s" locks))
245		(keys
246		 (message "Cancel-Key is %s" keys))))))))
247
248(provide 'canlock)
249
250;;; arch-tag: 033c4f09-b9f1-459d-bd0d-254430283f78
251;;; canlock.el ends here
252