bug#67012: 29.1; epa-sign-file pinentry loopback mode does not work with S/MIME

[Ulrich Mueller]

I was originally trying to sign e-mail messages with S/MIME using
mml-secure-sign-smime followed by message-send, which fails when I
customize epg-pinentry-mode as loopback.

The problem also occurs with epa-sign-file, which is easier to reproduce
(because it doesn't need as much configuration). So I am reporting the
bug for this command.

To reproduce, emacs -Q, then execute in the *scratch* buffer:

   (write-region "hello\n" nil "hello.txt")
   (require 'epa)

   (let ((epg-pinentry-mode 'loopback)
         (epa-protocol 'CMS))
     (epa-sign-file
      "hello.txt"
      (epa-select-keys (epg-make-context epa-protocol) "Key:" nil t)
      'normal))

This asks interactively to select a key. After doing so, it fails with
the following error (shown in an "*Error* (EPA Info)" buffer):

   Error while signing with "/usr/bin/gpgsm":

   gpgsm: ignoring gpg-agent inquiry 'PASSPHRASE'
   gpgsm: error creating signature: No passphrase given <GPG Agent>

Debugger *Backtrace* (key IDs x-ed out):

   Debugger entered--Lisp error: (epg-error "Sign failed" "")
     signal(epg-error ("Sign failed" ""))
     epa-sign-file("hello.txt" (#s(epg-key :owner-trust nil :sub-key-list 
(#s(epg-sub-key :validity nil :capability (encrypt sign) :secret-p nil 
:algorithm 1 :length 4096 :id "XXXXXXXXXXXXXXXX" :creation-time 20231107 
:expiration-time 20251106 :fingerprint 
"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX")) :user-id-list (#s(epg-user-id 
:validity nil :string (("CN" . "Ulrich Müller") ("OU" . "Institut fuer 
Kernphysik") ("O" . "Johannes Gutenberg-Universitaet Mainz") ("L" . "Mainz") 
("ST" . "Rheinland-Pfalz") ("C" . "DE")) :signature-list nil) #s(epg-user-id 
:validity nil :string "<ulm@uni-mainz.de>" :signature-list nil)))) normal)
     (let ((epg-pinentry-mode 'loopback) (epa-protocol 'CMS)) (epa-sign-file 
"hello.txt" (epa-select-keys (epg-make-context epa-protocol) "Key:" nil t) 
'normal))
     (progn (let ((epg-pinentry-mode 'loopback) (epa-protocol 'CMS)) 
(epa-sign-file "hello.txt" (epa-select-keys (epg-make-context epa-protocol) 
"Key:" nil t) 'normal)))
     eval((progn (let ((epg-pinentry-mode 'loopback) (epa-protocol 'CMS)) 
(epa-sign-file "hello.txt" (epa-select-keys (epg-make-context epa-protocol) 
"Key:" nil t) 'normal))) t)
     elisp--eval-last-sexp(t)
     eval-last-sexp(t)
     eval-print-last-sexp(nil)
     funcall-interactively(eval-print-last-sexp nil)
     call-interactively(eval-print-last-sexp nil nil)
     command-execute(eval-print-last-sexp)

When I change epg-pinentry-mode to ask or epa-protocol to OpenPGP in
the let-binding, things work as expected. In other words, only the
combination of S/MIME and pinentry loopback fails.

   |          | OpenPGP | CMS   |
   |----------+---------+-------|
   | ask      | works   | works |
   | loopback | works   | fails |

I use pinentry-gnome3, in case this should matter:

   $ readlink /usr/bin/pinentry
   pinentry-gnome3


In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.18.0) of 2023-10-24 built on localhost
Windowing system distributor 'The X.Org Foundation', version 11.0.12101009
System Description: Gentoo Linux

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