gpg: Make the use of "--verify FILE" for detached sigs harder.

* g10/openfile.c (open_sigfile): Factor some code out to ...
(get_matching_datafile): new function.
* g10/plaintext.c (hash_datafiles): Do not try to find matching file
in batch mode.
* g10/mainproc.c (check_sig_and_print): Print a warning if a possibly
matching data file is not used by a standard signatures.
--

Allowing to use the abbreviated form for detached signatures is a long
standing bug which has only been noticed by the public with the
release of 2.1.0.  :-(

What we do is to remove the ability to check detached signature in
--batch using the one file abbreviated mode.  This should exhibit
problems in scripts which use this insecure practice.  We also print a
warning if a matching data file exists but was not considered because
the detached signature was actually a standard signature:

  gpgv: Good signature from "Werner Koch (dist sig)"
  gpgv: WARNING: not a detached signature; \
  file 'gnupg-2.1.0.tar.bz2' was NOT verified!

We can only print a warning because it is possible that a standard
signature is indeed to be verified but by coincidence a file with a
matching name is stored alongside the standard signature.

Reported-by: Simon Nicolussi (to gnupg-users on Nov 7)
Signed-off-by: Werner Koch <[email protected]>

1 files changed, 16 insertions, 10 deletions

diff --git a/doc/gpg.texi b/doc/gpg.texi
index 499df8770..634b4e26b 100644
--- a/doc/gpg.texi
+++ b/doc/gpg.texi
@@ -214,16 +214,22 @@ files which don't begin with an encrypted message.
 
 @item --verify
 @opindex verify
-Assume that the first argument is a signed file or a detached signature
-and verify it without generating any output. With no arguments, the
-signature packet is read from STDIN. If only a sigfile is given, it may
-be a complete signature or a detached signature, in which case the
-signed stuff is expected in a file without the ".sig" or ".asc"
-extension.  With more than 1 argument, the first should be a detached
-signature and the remaining files are the signed stuff. To read the
-signed stuff from STDIN, use @samp{-} as the second filename.  For
-security reasons a detached signature cannot read the signed material
-from STDIN without denoting it in the above way.
+Assume that the first argument is a signed file and verify it without
+generating any output.  With no arguments, the signature packet is
+read from STDIN.  If only a one argument is given, it is expected to
+be a complete signature.
+
+With more than 1 argument, the first should be a detached signature
+and the remaining files ake up the the signed data. To read the signed
+data from STDIN, use @samp{-} as the second filename.  For security
+reasons a detached signature cannot read the signed material from
+STDIN without denoting it in the above way.
+
+Note: If the option @option{--batch} is not used, @command{@gpgname}
+may assume that a single argument is a file with a detached signature
+and it will try to find a matching data file by stripping certain
+suffixes.  Using this historical feature to verify a detached
+signature is strongly discouraged; always specify the data file too.
 
 Note: When verifying a cleartext signature, @command{gpg} verifies
 only what makes up the cleartext signed data and not any extra data