contrib: Add the matrix_decrypt helper.

This helper decrypts uploaded files and passes the file to a plumber
(rifle from ranger by default).

contrib/matrix_decrypt

@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+# matrix_decrypt - Download and decrypt an encrypted attachment
+# from a matrix server
+
+# Copyright © 2019 Damir Jelić <poljar@termina.org.uk>
+#
+# Permission to use, copy, modify, and/or distribute this software for
+# any purpose with or without fee is hereby granted, provided that the
+# above copyright notice and this permission notice appear in all copies.
+#
+# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+# SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER
+# RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF
+# CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+# CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+import argparse
+import requests
+import tempfile
+import subprocess
+
+from urllib.parse import urlparse, parse_qs
+from nio.crypto import decrypt_attachment
+
+
+def save_file(data):
+    """Save data to a temporary file and return its name."""
+    tmp_dir = tempfile.gettempdir()
+
+    with tempfile.NamedTemporaryFile(
+        prefix='plumber-',
+        dir=tmp_dir,
+        delete=False
+    ) as f:
+        f.write(data)
+        f.flush()
+        return f.name
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description='Download and decrypt matrix attachments'
+    )
+    parser.add_argument('url', help='the url of the attachment')
+    parser.add_argument('--plumber',
+                        help='program that gets called with the '
+                             'dowloaded file')
+
+    args = parser.parse_args()
+    url = urlparse(args.url)
+    query = parse_qs(url.query)
+
+    if not query["key"] or not query["iv"] or not query["hash"]:
+        print("Missing decryption argument")
+        return -1
+
+    key = query["key"][0]
+    iv = query["iv"][0]
+    hash = query["hash"][0]
+
+    http_url = "https://{}{}".format(url.netloc, url.path)
+
+    request = requests.get(http_url)
+
+    if not request.ok:
+        print("Error downloading file")
+        return -2
+
+    plumber = args.plumber or "/usr/bin/rifle"
+    plaintext = decrypt_attachment(request.content, key, hash, iv)
+    file_name = save_file(plaintext)
+
+    subprocess.run([plumber, "{file}".format(file=file_name)])
+
+    return 0
+
+
+if __name__ == "__main__":
+    main()