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

[Teemu Likonen]

Lars Ingebrigtsen [2019-07-12T16:22:41+02] wrote:

> Teemu Likonen <address@hidden> writes:
>> This commit enhances the feature to also use sender's email address
>> with GnuPG's (gpg) --sender option to clarify which user id made the
>> signature. The option is useful for two reasons when verifying the
>> signature:

> I think this makes sense, and the patch looks good. Perhaps this
> should also have a NEWS entry?

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

-- >8 --
Subject: [PATCH v2] 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             | 22 ++++++++++++++++++++++
 lisp/epg.el          |  8 ++++++++
 lisp/gnus/mml-sec.el |  9 +++++++--
 3 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 966bdda456..6ec036dd43 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1377,10 +1377,26 @@ 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
 
 ---
 *** 'epa-pinentry-mode' is renamed to 'epg-pinentry-mode'.
 It now applies to epg functions as well as epa functions.
@@ -1389,10 +1405,16 @@ It now applies to epg functions as well as epa 
functions.
 *** The alias functions 'epa--encode-coding-string',
 'epa--decode-coding-string', and 'epa--select-safe-coding-system' have
 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
 
 +++
 *** New user option 'rmail-output-reset-deleted-flag'.
 If this option is non-nil, messages appended to an output file by the
diff --git a/lisp/epg.el b/lisp/epg.el
index 8029bf5a93..ce58c520f1 100644
--- a/lisp/epg.el
+++ b/lisp/epg.el
@@ -206,10 +206,11 @@ 'epg-error
   compress-algorithm
   (passphrase-callback (list #'epg-passphrase-callback-function))
   progress-callback
   edit-callback
   signers
+  sender
   sig-notations
   process
   output-file
   result
   operation
@@ -1614,10 +1615,13 @@ epg-start-sign
                             (lambda (signer)
                               (list "-u"
                                     (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)
                         (list "--" (epg-data-file plain)))))
   ;; `gpgsm' does not read passphrase from stdin, so waiting is not needed.
@@ -1709,10 +1713,14 @@ epg-start-encrypt
                                         (epg-sub-key-id
                                          (car (epg-key-sub-key-list
                                                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
                            (mapcar
                             (lambda (recipient)
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
@@ -495,11 +495,12 @@ mml-secure-smime-encrypt-to-self
 
 (define-obsolete-variable-alias
   '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)
 
 (define-obsolete-variable-alias
   'mml-smime-sign-with-sender 'mml-secure-smime-sign-with-sender "25.1")
@@ -911,11 +912,13 @@ mml-secure-epg-encrypt
         (recipients (mml-secure-recipients protocol context config sender))
         (signer-names (mml-secure-signer-names protocol sender))
         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))
     (when (mml-secure-cache-passphrase-p protocol)
       (epg-context-set-passphrase-callback
@@ -942,10 +945,12 @@ mml-secure-epg-sign
         signature micalg)
     (when (eq 'OpenPGP protocol)
       (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
        (cons 'mml-secure-passphrase-callback protocol)))
     (condition-case error
-- 
2.20.1




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/  https://keybase.io/tlikonen  https://github.com/tlikonen

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