Re: [PATCH] MML/EPG: Add support for GnuPG's --sender option

[Teemu Likonen <tlikonen@iki.fi>]

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Teemu Likonen [2019-07-12T19:42:39+03] wrote:

> Below is a new version with NEWS entries. One entry is under Message and
> the other under EasyPG because this touches both.

One NEWS item was badly formatted. I'll try again...

-- >8 --
Subject: [PATCH v3] MML/EPG: Add support for GnuPG's --sender option

An already existing variable mml-secure-openpgp-sign-with-sender (if
non-nil) makes MML security to use message sender's email address to
find signer's key from GnuPG keyring.

This commit enhances the feature to also use sender's email address
with GnuPG's --sender option to clarify which user id made the
signature. The option is useful for two reasons when verifying the
signature:

  - GnuPG's TOFU statistics are updated for the specific user
    id (email) only

  - GnuPG's --auto-key-retrieve functionality can use WKD (web key
    directory) method for finding the signer's key.

Quotes from gpg(1) manual page (version 2.2.17):

    --auto-key-retrieve
    --no-auto-key-retrieve
           These options enable or disable the automatic retrieving of
           keys from a keyserver when verifying signatures made by
           keys that are not on the local keyring.  The default is
           --no-auto-key-retrieve.

           The order of methods tried to lookup the key is:

    [...]

           2.  If the signature has the Signer's UID set (e.g. using
           --sender while creating the signature) a Web Key
           Directory (WKD) lookup is done.  This is the default
           configuration but can be disabled by removing WKD from the
           auto-key-locate list or by using the option
           --disable-signer-uid.

    [...]

    --sender mbox
           This option has two purposes.  mbox must either be a
           complete user id with a proper mail address or just a mail
           address.  When creating a signature this option tells gpg
           the user id of a key used to make a signature if the key
           was not directly specified by a user id.  When verifying a
           signature the mbox is used to restrict the information
           printed by the TOFU code to matching user ids.
---
 etc/NEWS             | 23 +++++++++++++++++++++++
 lisp/epg.el          |  8 ++++++++
 lisp/gnus/mml-sec.el |  9 +++++++--
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 966bdda456..1a17e132c7 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1379,6 +1379,22 @@ are formatted as MIME digests.
 *** 'message-forward-included-headers' has changed its default to
 exclude most headers when forwarding.
 
+*** 'mml-secure-openpgp-sign-with-sender' sets also "gpg --sender"
+When 'mml-secure-openpgp-sign-with-sender' is non-nil message sender's
+email address (in addition to its old behaviour) will also be used to
+set gpg's "--sender email@domain" option.
+
+The option is useful for two reasons when verifying the signature:
+
+ 1. GnuPG's TOFU statistics are updated for the specific user id
+    (email) only. See gpg(1) man page about "--sender".
+
+ 2. GnuPG's --auto-key-retrieve functionality can use WKD (web key
+    directory) method for finding the signer's key. You need GnuPG
+    2.2.17 to fully benefit from this feature. See gpg(1) man page for
+    "--auto-key-retrieve".
+
+---
 ** EasyPG
 
 ---
@@ -1391,6 +1407,13 @@ It now applies to epg functions as well as epa functions.
 been removed.  Use 'encode-coding-string', 'decode-coding-string', and
 'select-safe-coding-system' instead.
 
+*** 'epg-context' structure supports now 'sender' slot
+The value of the new 'sender' slot (if a string) is used to set gpg's
+--sender option. This feature is used by
+'mml-secure-openpgp-sign-with-sender'. See gpg(1) manual page about
+"--sender" for more information.
+
+---
 ** Rmail
 
 +++
diff --git a/lisp/epg.el b/lisp/epg.el
index 8029bf5a93..ce58c520f1 100644
--- a/lisp/epg.el
+++ b/lisp/epg.el
@@ -208,6 +208,7 @@ 'epg-error
   progress-callback
   edit-callback
   signers
+  sender
   sig-notations
   process
   output-file
@@ -1616,6 +1617,9 @@ epg-start-sign
 				     (epg-sub-key-id
 				      (car (epg-key-sub-key-list signer)))))
 			     (epg-context-signers context)))
+                     (let ((sender (epg-context-sender context)))
+                       (when (stringp sender)
+                         (list "--sender" sender)))
 		     (epg--args-from-sig-notations
 		      (epg-context-sig-notations context))
 		     (if (epg-data-file plain)
@@ -1711,6 +1715,10 @@ epg-start-encrypt
 						signer)))))
 				 (epg-context-signers context))))
 		     (if sign
+                         (let ((sender (epg-context-sender context)))
+                           (when (stringp sender)
+                             (list "--sender" sender))))
+                     (if sign
 			 (epg--args-from-sig-notations
 			  (epg-context-sig-notations context)))
 		     (apply #'nconc
diff --git a/lisp/gnus/mml-sec.el b/lisp/gnus/mml-sec.el
index 02a27b367c..07d2028534 100644
--- a/lisp/gnus/mml-sec.el
+++ b/lisp/gnus/mml-sec.el
@@ -497,7 +497,8 @@ mml-secure-smime-encrypt-to-self
   'mml2015-sign-with-sender 'mml-secure-openpgp-sign-with-sender "25.1")
 ;mml1991-sign-with-sender did never exist.
 (defcustom mml-secure-openpgp-sign-with-sender nil
-  "If t, use message sender to find an OpenPGP key to sign with."
+  "If t, use message sender to find an OpenPGP key to sign with.
+Also use message's sender with GnuPG's --sender option."
   :group 'mime-security
   :type 'boolean)
 
@@ -913,7 +914,9 @@ mml-secure-epg-encrypt
 	 cipher signers)
     (when sign
       (setq signers (mml-secure-signers context signer-names))
-      (setf (epg-context-signers context) signers))
+      (setf (epg-context-signers context) signers)
+      (when mml-secure-openpgp-sign-with-sender
+        (setf (epg-context-sender context) sender)))
     (when (eq 'OpenPGP protocol)
       (setf (epg-context-armor context) t)
       (setf (epg-context-textmode context) t))
@@ -944,6 +947,8 @@ mml-secure-epg-sign
       (setf (epg-context-armor context) t)
       (setf (epg-context-textmode context) t))
     (setf (epg-context-signers context) signers)
+    (when mml-secure-openpgp-sign-with-sender
+      (setf (epg-context-sender context) sender))
     (when (mml-secure-cache-passphrase-p protocol)
       (epg-context-set-passphrase-callback
        context
-- 
2.20.1

-- 
///  OpenPGP key: 4E1055DC84E9DFF613D78557719D69D324539450
//  https://keys.openpgp.org/search?q=tlikonen@iki.fi
/  https://keybase.io/tlikonen  https://github.com/tlikonen

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