example: verify signatures

* Added example for verifying detached signatures against the files
  they're the signatures for.

lang/python/examples/howto/verify-signatures.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+from __future__ import absolute_import, division, unicode_literals
+
+# Copyright (C) 2018 Ben McGinnes <ben@gnupg.org>
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 2 of the License, or (at your option) any later
+# version.
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the Free
+# Software Foundation; either version 2.1 of the License, or (at your option)
+# any later version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License and the GNU
+# Lesser General Public Licensefor more details.
+#
+# You should have received a copy of the GNU General Public License and the GNU
+# Lesser General Public along with this program; if not, see
+# <http://www.gnu.org/licenses/>.
+
+import gpg
+import sys
+import time
+
+"""
+Verifies a signed file which has been signed with a detached signature.
+"""
+
+if len(sys.argv) > 2:
+    filename = sys.argv[1]
+    sig_file = sys.argv[2]
+elif len(sys.argv) == 2:
+    filename = sys.argv[1]
+    sig_file = input("Enter the path and filename of the detached signature: ")
+else:
+    filename = input("Enter the path and filename to verify: ")
+    sig_file = input("Enter the path and filename of the detached signature: ")
+
+c = gpg.Context()
+
+try:
+    data, result = c.verify(open(filename), open(sig_file))
+    verified = True
+except gpg.errors.BadSignatures as e:
+    verified = False
+    print(e)
+
+if verified is True:
+    for i in range(len(result.signatures)):
+        sign = result.signatures[i]
+        print("""Good signature from:
+{0}
+with key {1}
+made at {2}
+""".format(c.get_key(sign.fpr).uids[0].uid, sign.fpr,
+           time.ctime(sign.timestamp)))
+else:
+    pass