gpg: Add new usage option for drop-subkey filters.fix-T4019

* g10/import.c (impex_filter_getval): Add new "usage" property for
drop-subkey filter.
--

For example, this permits extraction of only encryption-capable
subkeys like so:

    gpg --export-filter 'drop-subkey=usage !~ e' --export $FPR

GnuPG-Bug-id: 4019
Signed-off-by: Daniel Kahn Gillmor <[email protected]>

2 files changed, 15 insertions, 0 deletions

diff --git a/doc/gpg.texi b/doc/gpg.texi
index 2915d25e1..d0aa0100e 100644
--- a/doc/gpg.texi
+++ b/doc/gpg.texi
@@ -2418,6 +2418,11 @@ The available properties are:
   Boolean indicating whether a key or subkey is a secret one.
   (drop-subkey)
 
+  @item usage
+  A string indicating the usage flags for the subkey, from the
+  sequence ``ecsa?''.  For example, a subkey capable of just signing
+  and authentication would be an exact match for ``sa''. (drop-subkey)
+
   @item sig_created
   @itemx sig_created_d
   The first is the timestamp a signature packet was created.  The
diff --git a/g10/import.c b/g10/import.c
index 6d2beaf33..b6096608e 100644
--- a/g10/import.c
+++ b/g10/import.c
@@ -1296,6 +1296,16 @@ impex_filter_getval (void *cookie, const char *propname)
         {
           result = pk_is_disabled (pk)? "1":"0";
         }
+      else if (!strcmp (propname, "usage"))
+        {
+          snprintf (numbuf, sizeof numbuf, "%s%s%s%s%s",
+                    (pk->pubkey_usage & PUBKEY_USAGE_ENC)?"e":"",
+                    (pk->pubkey_usage & PUBKEY_USAGE_SIG)?"s":"",
+                    (pk->pubkey_usage & PUBKEY_USAGE_CERT)?"c":"",
+                    (pk->pubkey_usage & PUBKEY_USAGE_AUTH)?"a":"",
+                    (pk->pubkey_usage & PUBKEY_USAGE_UNKNOWN)?"?":"");
+          result = numbuf;
+        }
       else
         result = NULL;
     }