bug#65316: 29.1.50; EPA can falsely report "wrong passphrase" when decryption fails

bug#65316: 29.1.50; EPA can falsely report "wrong passphrase" when decryption fails

[Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Placeholder to draw a bug number ... details and patch to follow.

--------------------------------------------------

In GNU Emacs 29.1.50 (build 6, x86_64-pc-linux-gnu, GTK+ Version
 3.24.24, cairo version 1.16.0) of 2023-08-15 built on sappc2
Repository revision: 26949819df07cb36ed773cc9df6ca91f5f49e3d5
Repository branch: false-positives-from-epa--wrong-password-p
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure -C --with-native-compilation'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  value of $LC_COLLATE: POSIX
  value of $LC_TIME: POSIX
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

bug#65316: 29.1.50; EPA can falsely report "wrong passphrase" when decryption fails

[Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors]

[-- Attachment #1: Type: text/plain, Size: 3134 bytes --]

Package: epa.el
Tags: patch

Repro Case:

# prepare a public-key-encrypted file test.gpg in cwd

# make pinentry executable non-executable
sudo chmod a-x /usr/bin/pinentry

emacs -Q

C-x C-f test.gpg RET

=> Wrong passphrase: No secret key

Where Emacs 27 would report in a separate *Error* buffer:

----------------------------------------
Error while decrypting with "/usr/bin/gpg":

gpg: encrypted with 3072-bit RSA key, ID D0EB77D91C0802D6, created 2022-12-03
      "test-key"
gpg: public key decryption failed: No pinentry
gpg: decryption failed: No secret key
----------------------------------------

The root cause is in function `epa--wrong-password-p', defined as
follows:

----------------------------------------
(defun epa--wrong-password-p (context)
  (let ((error-string (epg-context-error-output context)))
    (and (string-match
          "decryption failed: \\(Bad session key\\|No secret key\\)"
          error-string)
         (match-string 1 error-string))))
----------------------------------------

It should not search for "No secret key" but rather for "Bad
passphrase".  "No secret key" just means that there is no secret key
available to decrypt the file, "Bad passphrase" means that no secret
keys can be used because of a wrong passphrase.

I collected a couple of non-bad-passphrase error messages from GnuPG
decryption failures, all done with:

  [emacs-29]$ gpg --version
  gpg (GnuPG) 2.2.27
  libgcrypt 1.8.8

----------------------------------------
# public key, chmod a-x /usr/bin/pinentry
gpg: encrypted with 3072-bit RSA key, ID D0EB77D91C0802D6, created 2022-12-03
      "test-key"
gpg: public key decryption failed: No pinentry
gpg: decryption failed: No secret key

# symmetric, chmod a-x /usr/bin/pinentry
gpg: AES256.CFB encrypted data
gpg: problem with the agent: No pinentry
gpg: encrypted with 1 passphrase
gpg: decryption failed: No secret key

# public key, 0744 empty GnuPG home directory
gpg: WARNING: unsafe permissions on homedir '/home/jschmidt/work/emacs-29/xxx'
gpg: encrypted with RSA key, ID D0EB77D91C0802D6
gpg: decryption failed: No secret key

# public key, 0400 empty GnuPG home directory
gpg: failed to create temporary file '/home/jschmidt/work/emacs-29/xxx/.#lk0x00005571263a1230.sappc2.4974': Permission denied
gpg: keyblock resource '/home/jschmidt/work/emacs-29/xxx/pubring.kbx': Permission denied
gpg: encrypted with RSA key, ID D0EB77D91C0802D6
gpg: decryption failed: No secret key

# public key, 0700 empty GnuPG home directory
gpg: encrypted with RSA key, ID D0EB77D91C0802D6
gpg: decryption failed: No secret key
----------------------------------------

And here the real bad-passphrase messages:

----------------------------------------
# symmetric, bad passphrase entered
gpg: AES256.CFB encrypted data
gpg: encrypted with 1 passphrase
gpg: decryption failed: Bad session key

# public key, bad passphrase entered
gpg: encrypted with 3072-bit RSA key, ID D0EB77D91C0802D6, created 2022-12-03
      "test-key"
gpg: public key decryption failed: Bad passphrase
gpg: decryption failed: No secret key
----------------------------------------

Patch attached.

[-- Attachment #2: 0001-Avoid-false-wrong-passphrase-messages-in-EPA.patch --]
[-- Type: text/x-patch, Size: 1470 bytes --]

From 17f4e8a888534ffc783b3a6088229d4355afde30 Mon Sep 17 00:00:00 2001
From: Jens Schmidt <jschmidt4gnu@vodafonemail.de>
Date: Tue, 15 Aug 2023 21:37:08 +0200
Subject: [PATCH] Avoid false "wrong passphrase" messages in EPA

* lisp/epa-file.el (epa--wrong-password-p): Use a stricter regexp to
match "wrong passphrase" errors generated by GnuPG.  (Bug#65316)
---
 lisp/epa-file.el | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lisp/epa-file.el b/lisp/epa-file.el
index 4d8ca11e809..a27f241c0c3 100644
--- a/lisp/epa-file.el
+++ b/lisp/epa-file.el
@@ -123,9 +123,16 @@ epa-file--find-file-not-found-function
 	      (cons "Opening input file" (cdr error))))))
 
 (defun epa--wrong-password-p (context)
+  "Return whether a wrong password caused the error in CONTEXT."
   (let ((error-string (epg-context-error-output context)))
+    ;; Use a strict regexp here that really only matches "wrong
+    ;; passphrase" errors to avoid hiding diagnostic information
+    ;; (bug#65316).  Below regexp also can fail to match non-English
+    ;; messages, since at least the "decryption failed" part of it
+    ;; seems to be localized.  But since this means false negatives
+    ;; this is probably OK.
     (and (string-match
-          "decryption failed: \\(Bad session key\\|No secret key\\)"
+          "decryption failed: \\(Bad session key\\|Bad passphrase\\)"
           error-string)
          (match-string 1 error-string))))
 
-- 
2.30.2

bug#65316: 29.1.50; EPA can falsely report "wrong passphrase" when decryption fails

[Eli Zaretskii]

> Date: Tue, 15 Aug 2023 21:49:07 +0200
> From:  Jens Schmidt via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> # prepare a public-key-encrypted file test.gpg in cwd
> 
> # make pinentry executable non-executable
> sudo chmod a-x /usr/bin/pinentry
> 
> emacs -Q
> 
> C-x C-f test.gpg RET
> 
> => Wrong passphrase: No secret key
> 
> Where Emacs 27 would report in a separate *Error* buffer:
> 
> ----------------------------------------
> Error while decrypting with "/usr/bin/gpg":
> 
> gpg: encrypted with 3072-bit RSA key, ID D0EB77D91C0802D6, created 2022-12-03
>       "test-key"
> gpg: public key decryption failed: No pinentry
> gpg: decryption failed: No secret key
> ----------------------------------------
> 
> The root cause is in function `epa--wrong-password-p', defined as
> follows:
> 
> ----------------------------------------
> (defun epa--wrong-password-p (context)
>   (let ((error-string (epg-context-error-output context)))
>     (and (string-match
>           "decryption failed: \\(Bad session key\\|No secret key\\)"
>           error-string)
>          (match-string 1 error-string))))
> ----------------------------------------
> 
> It should not search for "No secret key" but rather for "Bad
> passphrase".  "No secret key" just means that there is no secret key
> available to decrypt the file, "Bad passphrase" means that no secret
> keys can be used because of a wrong passphrase.
> 
> I collected a couple of non-bad-passphrase error messages from GnuPG
> decryption failures, all done with:
> 
>   [emacs-29]$ gpg --version
>   gpg (GnuPG) 2.2.27
>   libgcrypt 1.8.8
> 
> ----------------------------------------
> # public key, chmod a-x /usr/bin/pinentry
> gpg: encrypted with 3072-bit RSA key, ID D0EB77D91C0802D6, created 2022-12-03
>       "test-key"
> gpg: public key decryption failed: No pinentry
> gpg: decryption failed: No secret key
> 
> # symmetric, chmod a-x /usr/bin/pinentry
> gpg: AES256.CFB encrypted data
> gpg: problem with the agent: No pinentry
> gpg: encrypted with 1 passphrase
> gpg: decryption failed: No secret key
> 
> # public key, 0744 empty GnuPG home directory
> gpg: WARNING: unsafe permissions on homedir '/home/jschmidt/work/emacs-29/xxx'
> gpg: encrypted with RSA key, ID D0EB77D91C0802D6
> gpg: decryption failed: No secret key
> 
> # public key, 0400 empty GnuPG home directory
> gpg: failed to create temporary file '/home/jschmidt/work/emacs-29/xxx/.#lk0x00005571263a1230.sappc2.4974': Permission denied
> gpg: keyblock resource '/home/jschmidt/work/emacs-29/xxx/pubring.kbx': Permission denied
> gpg: encrypted with RSA key, ID D0EB77D91C0802D6
> gpg: decryption failed: No secret key
> 
> # public key, 0700 empty GnuPG home directory
> gpg: encrypted with RSA key, ID D0EB77D91C0802D6
> gpg: decryption failed: No secret key
> ----------------------------------------
> 
> And here the real bad-passphrase messages:
> 
> ----------------------------------------
> # symmetric, bad passphrase entered
> gpg: AES256.CFB encrypted data
> gpg: encrypted with 1 passphrase
> gpg: decryption failed: Bad session key
> 
> # public key, bad passphrase entered
> gpg: encrypted with 3072-bit RSA key, ID D0EB77D91C0802D6, created 2022-12-03
>       "test-key"
> gpg: public key decryption failed: Bad passphrase
> gpg: decryption failed: No secret key
> ----------------------------------------
> 
> Patch attached.

Thanks, installed on the emacs-29 branch, and closing the bug.