python bindings: export secret keys

* The holy grail: a function to export secret keys.
* GPGME will still invoke pinentry and gpg-agent as usual to authorise
  the export.
* Mostly similar to the two previous export functions for public keys
  except that it will return None if the result had a length of zero
  bytes.  Meaning that the difference between the specified pattern
  (if any) not matching available keys and an incorrect passphrase is
  not able to be determined from this function (or the underlying one
  for that matter).

Signed-off-by: Ben McGinnes <ben@adversary.org>

lang/python/src/core.py

@@ -635,6 +635,47 @@ class Context(GpgmeWrapper):
 
         return result
 
+    def key_export_secret(self, pattern=None):
+        """Export secret keys.
+
+        Exports secret keys matching the pattern specified.  If no
+        pattern is specified then exports or attempts to export all
+        available secret keys.
+
+        IMPORTANT: Each secret key to be exported will prompt for its
+        passphrase via an invocation of pinentry and gpg-agent.  If the
+        passphrase is not entered or does not match then no data will be
+        exported.  This is the same result as when specifying a pattern
+        that is not matched by the available keys.
+
+        Keyword arguments:
+        pattern	-- return keys matching pattern (default: all keys)
+
+        Returns:
+                -- On success a key block containing one or more OpenPGP
+                   secret keys in either ASCII armoured or binary format
+                   as determined by the Context().
+                -- On failure while not raising an exception, returns None.
+
+        Raises:
+        GPGMEError     -- as signaled by the underlying library.
+        """
+        data = Data()
+        mode = gpgme.GPGME_EXPORT_MODE_SECRET
+        try:
+            self.op_export(pattern, mode, data)
+            data.seek(0, os.SEEK_SET)
+            sk_result = data.read()
+        except GPGMEError as e:
+            sk_result = e
+
+        if len(sk_result) > 0:
+            result = sk_result
+        else:
+            result = None
+
+        return result
+
     def keylist(self, pattern=None, secret=False,
                 mode=constants.keylist.mode.LOCAL,
                 source=None):