Weaknesses in PGP signatures

Weaknesses in PGP signatures

[Steven L. Baur]

-----BEGIN PGP SIGNED MESSAGE-----

There's recently been a discussion of forging techniques that are hard
to repudiate when PGP signatures are used (alt.security.pgp,
sci.crypt, and the cypherpunks mailing list).  A suggested possible
solution to make this harder is to include some of the mail headers
into the PGP signed portion of the message.

A Gnus implementation of this follows:
;;; Pgp signed messages are vulnerable to various kinds of badness due to
;;; the separation of header information.  Fix it.

(defconst gnus-pgp-included-headers '("From"
				      "To"
				      "Newsgroups"
				      "Message-ID"
				      "Date"
				      "Subject"
				      "Cc"
				      "Gcc")
  "Headers to include in signed portion of PGP signed message.")

(defconst gnus-pgp-signed-headers "----BEGIN PGP SIGNED HEADERS----\n"
  "String to use for separation in message.")

(defun gnus-article-sign-message (arg)
  "Sign a message with PGP, including outgoing headers in an included
block, as per the suggestion of \"Dr. Dimitri Vulis\" <dlv@bwalk.dm.com>."
  (interactive "p")
  (save-excursion
    (save-restriction
      (gnus-inews-narrow-to-headers)
      (goto-char (point-max))

      (or (mail-fetch-field "date")
	  (insert (concat "Date: " (gnus-inews-date) "\n")))
      (or (mail-fetch-field "message-id")
	  (insert (concat "Message-ID: " (gnus-inews-message-id) "\n")))))

  (save-excursion
    (goto-char (point-max))
    (insert "\n")
    ;; If there is already a header block (eg. after undoing a signature)
    ;; remove it entirely, and rebuild from scratch.
    (if (re-search-backward gnus-pgp-signed-headers nil t)
	  (kill-region (point) (point-max)))
    (insert gnus-pgp-signed-headers)
    (let ((headers gnus-pgp-included-headers)
	  header header-value)
      (while (setq header (car headers))
	(setq headers (cdr headers))
	(save-excursion
	  (save-restriction
	  (gnus-narrow-to-headers)
	  (setq header-value (mail-fetch-field header))))
	(if header-value
	    (insert (concat header ": " header-value "\n"))))
      (insert "\n")))
  (mc-sign arg))

- -- 
steve@miranova.com baur

- ----BEGIN PGP SIGNED HEADERS----
To: ding@ifi.uio.no
Message-ID: <m2ka3em7v9.fsf@diana.miranova.com>
Date: 30 Dec 1995 15:47:54 -0800
Subject: Weaknesses in PGP signatures


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