Symmetric encryption example.

* lang/python/examples/howto/symcrypt-file.py: A variation on standard
  key based encryption.

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

@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+from __future__ import absolute_import, division, unicode_literals
+
+import gpg
+import sys
+
+# 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/>.
+
+"""
+Symmetrically encrypts a file.  Passphrase will be prompted for via Pinentry.
+
+Will produce both an ASCII armoured and GPG binary format copy of the encrypted
+file.
+"""
+
+if len(sys.argv) > 2:
+    filename = " ".join(sys.argv[1:])
+elif len(sys.argv) == 2:
+    filename = sys.argv[1]
+else:
+    filename = input("Enter the path and filename to encrypt: ")
+
+with open(filename, "rb") as f:
+    text = f.read()
+
+with gpg.Context(armor=True) as ca:
+    try:
+        ciphertext, result, sign_result = ca.encrypt(text, passphrase=None,
+                                                     sign=False)
+        with open("{0}.asc".format(filename), "wb") as fa:
+            fa.write(ciphertext)
+    except gpg.errors.InvalidRecipients as e:
+        print(e)
+
+with gpg.Context() as cg:
+    try:
+        ciphertext, result, sign_result = cg.encrypt(text, passphrase=None,
+                                                     sign=False)
+        with open("{0}.gpg".format(filename), "wb") as fg:
+            fg.write(ciphertext)
+    except gpg.errors.InvalidRecipients as e:
+        print(e)