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;;; canlock.el --- functions for Cancel-Lock feature
;; Copyright (C) 1998-1999, 2001-2021 Free Software Foundation, Inc.
;; Author: Katsumi Yamaoka <yamaoka@jpl.org>
;; Keywords: news, cancel-lock, hmac, sha1, rfc2104
;; This file is part of GNU Emacs.
;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
;;; Commentary:
;; Canlock is a library for generating and verifying Cancel-Lock and/or
;; Cancel-Key header in news articles. This is used to protect articles
;; from rogue cancel, supersede or replace attacks. The method is based
;; on draft-ietf-usefor-cancel-lock-01.txt which was released on November
;; 3rd 1998. For instance, you can add Cancel-Lock (and possibly Cancel-
;; Key) header in a news article by using a hook which will be evaluated
;; just before sending an article as follows:
;;
;; (add-hook '*e**a*e-header-hook 'canlock-insert-header t)
;;
;; Verifying Cancel-Lock is mainly a function of news servers, however,
;; you can verify your own article using the command `canlock-verify' in
;; the (raw) article buffer. You will be prompted for the password for
;; each time if the option `canlock-password' or
;; `canlock-password-for-verify' is nil. Note that setting these
;; options is a bit unsafe.
;;; Code:
(require 'sha1)
(defvar mail-header-separator)
(defgroup canlock nil
"The Cancel-Lock feature."
:group 'news)
(defcustom canlock-password nil
"Password to use when signing a Cancel-Lock or a Cancel-Key header."
:type '(radio (const :format "Not specified " nil)
(string :tag "Password"))
:group 'canlock)
(defcustom canlock-password-for-verify canlock-password
"Password to use when verifying a Cancel-Lock or a Cancel-Key header."
:type '(radio (const :format "Not specified " nil)
(string :tag "Password"))
:group 'canlock)
(defcustom canlock-force-insert-header nil
"If non-nil, insert a Cancel-Lock or a Cancel-Key header even if the
buffer does not look like a news message."
:type 'boolean
:group 'canlock)
(defun canlock-sha1 (message)
"Make a SHA-1 digest of MESSAGE as a unibyte string of length 20 bytes."
(sha1 message nil nil 'binary))
(defun canlock-make-cancel-key (message-id password)
"Make a Cancel-Key header."
(when (> (length password) 20)
(setq password (canlock-sha1 password)))
(setq password (concat password (make-string (- 64 (length password)) 0)))
(let ((ipad (mapconcat (lambda (byte)
(char-to-string (logxor 54 byte)))
password ""))
(opad (mapconcat (lambda (byte)
(char-to-string (logxor 92 byte)))
password "")))
(base64-encode-string
(canlock-sha1 (concat opad (canlock-sha1 (concat ipad message-id)))))))
(defun canlock-narrow-to-header ()
"Narrow the buffer to the head of the message."
(let (case-fold-search)
(narrow-to-region
(goto-char (point-min))
(goto-char (if (re-search-forward
(format "^$\\|^%s$"
(regexp-quote mail-header-separator))
nil t)
(match-beginning 0)
(point-max))))))
(defun canlock-delete-headers ()
"Delete Cancel-Key or Cancel-Lock headers in the narrowed buffer."
(let ((case-fold-search t))
(goto-char (point-min))
(while (re-search-forward "^Cancel-\\(Key\\|Lock\\):" nil t)
(delete-region (match-beginning 0)
(if (re-search-forward "^[^\t ]" nil t)
(goto-char (match-beginning 0))
(point-max))))))
(defun canlock-fetch-fields (&optional key)
"Return a list of the values of Cancel-Lock header.
If KEY is non-nil, look for a Cancel-Key header instead. The buffer
is expected to be narrowed to just the headers of the message."
(let ((field (mail-fetch-field (if key "Cancel-Key" "Cancel-Lock")))
fields rest
(case-fold-search t))
(when field
(setq fields (split-string field "[\t\n\r ,]+"))
(while fields
(when (string-match "^sha1:" (setq field (pop fields)))
(push (substring field 5) rest)))
(nreverse rest))))
(defun canlock-fetch-id-for-key ()
"Return a Message-ID in Cancel, Supersedes or Replaces header.
The buffer is expected to be narrowed to just the headers of the
message."
(or (let ((cancel (mail-fetch-field "Control")))
(and cancel
(string-match "^cancel[\t ]+\\(<[^\t\n @<>]+@[^\t\n @<>]+>\\)"
cancel)
(match-string 1 cancel)))
(mail-fetch-field "Supersedes")
(mail-fetch-field "Replaces")))
;;;###autoload
(defun canlock-insert-header (&optional id-for-key id-for-lock password)
"Insert a Cancel-Key and/or a Cancel-Lock header if possible."
(let (news control key-for-key key-for-lock)
(save-excursion
(save-restriction
(canlock-narrow-to-header)
(when (setq news (or canlock-force-insert-header
(mail-fetch-field "Newsgroups")))
(unless id-for-key
(setq id-for-key (canlock-fetch-id-for-key)))
(if (and (setq control (mail-fetch-field "Control"))
(string-match "^cancel[\t ]+<[^\t\n @<>]+@[^\t\n @<>]+>"
control))
(setq id-for-lock nil)
(unless id-for-lock
(setq id-for-lock (mail-fetch-field "Message-ID"))))
(canlock-delete-headers)
(goto-char (point-max))))
(when news
(if (not (or id-for-key id-for-lock))
(message "There are no Message-ID(s)")
(unless password
(setq password (or canlock-password
(read-passwd
"Password for Canlock: "))))
(if (or (not (stringp password)) (zerop (length password)))
(message "Password for Canlock is bad")
(setq key-for-key (when id-for-key
(canlock-make-cancel-key
id-for-key password))
key-for-lock (when id-for-lock
(canlock-make-cancel-key
id-for-lock password)))
(if (not (or key-for-key key-for-lock))
(message "Couldn't insert Canlock header")
(when key-for-key
(insert "Cancel-Key: sha1:" key-for-key "\n"))
(when key-for-lock
(insert "Cancel-Lock: sha1:"
(base64-encode-string (canlock-sha1 key-for-lock))
"\n")))))))))
;;;###autoload
(defun canlock-verify (&optional buffer)
"Verify Cancel-Lock or Cancel-Key in BUFFER.
If BUFFER is nil, the current buffer is assumed. Signal an error if
it fails."
(interactive)
(let (keys locks errmsg id-for-key id-for-lock password
key-for-key key-for-lock match)
(save-excursion
(when buffer
(set-buffer buffer))
(save-restriction
(widen)
(canlock-narrow-to-header)
(setq keys (canlock-fetch-fields 'key)
locks (canlock-fetch-fields))
(if (not (or keys locks))
(setq errmsg
"There are neither Cancel-Lock nor Cancel-Key headers")
(setq id-for-key (canlock-fetch-id-for-key)
id-for-lock (mail-fetch-field "Message-ID"))
(or id-for-key id-for-lock
(setq errmsg "There are no Message-ID(s)")))))
(if errmsg
(error "%s" errmsg)
(setq password (or canlock-password-for-verify
(read-passwd "Password for Canlock: ")))
(if (or (not (stringp password)) (zerop (length password)))
(error "Password for Canlock is bad")
(when keys
(when id-for-key
(setq key-for-key (canlock-make-cancel-key id-for-key password))
(while (and keys (not match))
(setq match (string-equal key-for-key (pop keys)))))
(setq keys (if match "good" "bad")))
(setq match nil)
(when locks
(when id-for-lock
(setq key-for-lock
(base64-encode-string
(canlock-sha1 (canlock-make-cancel-key id-for-lock
password))))
(when (and locks (not match))
(setq match (string-equal key-for-lock (pop locks)))))
(setq locks (if match "good" "bad")))
(prog1
(when (member "bad" (list keys locks))
"bad")
(cond ((and keys locks)
(message "Cancel-Key is %s, Cancel-Lock is %s" keys locks))
(locks
(message "Cancel-Lock is %s" locks))
(keys
(message "Cancel-Key is %s" keys))))))))
(provide 'canlock)
;;; canlock.el ends here
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