g10: Skip signing keys where no secret key is available.

* g10/getkey.c (finish_lookup): When requiring PUBKEY_USAGE_SIG, skip
over keys where no signing key is available.

--

This should only be relevant when gpg is required to choose which key
to sign with -- if verifying signatures, we already know which subkey
to look at, and indeed gpg doesn't seem to have a problem with this.

This patch comes from
https://bugs.gnupg.org/gnupg/file793/sign-fix.patch

I (dkg) have reviewed and tested it with missing local keys, and it
makes sense to me as the default behavior.  If the user has the secret
key for a signing-capable subkey available and the command is --sign,
it should be used.

If the user has explicitly specified a subkey that happens to be
missing (e.g. with the trailing ! for --default-key 0x${FPR}!) then
this does not override that behavior (the signature will still fail).

GnuPG-bug-id: 1967
Debian-bug-id: 834922

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Gbp-Pq: Topic skip-missing-signing-keys
Gbp-Pq: Name 0076-g10-Skip-signing-keys-where-no-secret-key-is-availab.patch

diff --git a/g10/getkey.c b/g10/getkey.c
index 961d7de226f979f009f5d744f3c4a81195acdaa6..bb31dfbe0b17da8c174d9f6e6d379670f57327d9 100644 (file)
--- a/g10/getkey.c
+++ b/g10/getkey.c
@@ -3529,6 +3529,13 @@ finish_lookup (kbnode_t keyblock, unsigned int req_usage, int want_exact,
              continue;
            }
 
+         if ((req_usage & PUBKEY_USAGE_SIG) && agent_probe_secret_key (NULL, pk))
+           {
+             if (DBG_LOOKUP)
+               log_debug ("\tno secret key for signing\n");
+             continue;
+           }
+
          if (DBG_LOOKUP)
            log_debug ("\tsubkey might be fine\n");
          /* In case a key has a timestamp of 0 set, we make sure