example: sign file

* Similar to encrypt file except for signing a file in normal mode.
* Noticed additional changes to be made to the howto to match this,
  but they will have to wait due to a power outage (currently running
  on battery and a mobile connection, but that won't last).

lang/python/examples/howto/sign-file.py

@@ -0,0 +1,63 @@
+#!/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
+
+"""
+Signs a file with a specified key.  If entering both the key and the filename
+on the command line, the key must be entered first.
+
+Will produce both an ASCII armoured and GPG binary format copy of the encrypted file.
+"""
+
+if len(sys.argv) > 3:
+    logrus = sys.argv[1]
+    filename = " ".join(sys.argv[2:])
+elif len(sys.argv) == 3:
+    logrus = sys.argv[1]
+    filename = sys.argv[2]
+elif len(sys.argv) == 2:
+    logrus = sys.argv[1]
+    filename = input("Enter the path and filename to sign: ")
+else:
+    logrus = input("Enter the fingerprint or key ID to sign with: ")
+    filename = input("Enter the path and filename to sign: ")
+
+with open(filename, "rb") as f:
+    text = f.read()
+
+key = list(gpg.Context().keylist(pattern=logrus))
+
+with gpg.Context(armor=True, signers=key) as ca:
+    signed_data, result = ca.sign(text, mode=gpg.constants.sig.mode.NORMAL)
+    with open("{0}.asc".format(filename), "wb") as fa:
+        fa.write(signed_data)
+
+with gpg.Context(signers=key) as cg:
+    signed_data, result = cg.sign(text, mode=gpg.constants.sig.mode.NORMAL)
+    with open("{0}.gpg".format(filename), "wb") as fg:
+        fg.write(signed_data)