Add filter verifing DKIM signature

Filter tags e-mail having DKIM header with either `dkim-ok` or
`dkim-fail`.

afew/filters/DKIMValidityFilter.py

@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+# SPDX-License-Identifier: ISC
+# Copyright (c) Amadeusz Zolnowski <aidecoe@aidecoe.name>
+
+from __future__ import print_function, absolute_import, unicode_literals
+
+import dkim
+
+from .BaseFilter import Filter
+
+
+def verify_dkim(path):
+    '''
+    Verify DKIM signature of an e-mail file.
+
+    :param path: Path to the e-mail file.
+    :returns: Whether DKIM signature is valid or not.
+    '''
+    with open(path, 'rb') as message_file:
+        message_bytes = message_file.read()
+
+    return dkim.verify(message_bytes)
+
+
+class DKIMValidityFilter(Filter):
+    '''
+    Verifies DKIM signature of an e-mail which has DKIM header.
+    '''
+    message = 'Verify DKIM signature'
+    header = 'DKIM-Signature'
+
+    def __init__(self, database, ok_tag='dkim-ok', fail_tag='dkim-fail'):
+        super(DKIMValidityFilter, self).__init__(database)
+        self.dkim_tag = {True: ok_tag, False: fail_tag}
+
+    def handle_message(self, message):
+        if message.get_header(self.header):
+            dkim_ok = all(map(verify_dkim, message.get_filenames()))
+            self.add_tags(message, self.dkim_tag[dkim_ok])

afew/tests/test_settings.py

@@ -27,6 +27,7 @@ def test_all_builtin_FilterRegistrys_exist(self):
         from afew import FilterRegistry
         self.assertEqual(sorted(['FolderNameFilter',
                            'ArchiveSentMailsFilter',
+                           'DKIMValidityFilter',
                            'DMARCReportInspectionFilter',
                            'InboxFilter',
                            'SpamFilter',

setup.py

@@ -11,6 +11,7 @@ def get_requires():
     if os.environ.get('TRAVIS') != 'true':
         yield 'notmuch'
     yield 'chardet'
+    yield 'dkimpy'
 
 setup(
     name='afew',
@@ -30,6 +31,7 @@ def get_requires():
         'afew.filter': [
             'Filter = afew.filters.BaseFilter:Filter',
             'ArchiveSentMailsFilter = afew.filters.ArchiveSentMailsFilter:ArchiveSentMailsFilter',
+            'DKIMValidityFilter = afew.filters.DKIMValidityFilter:DKIMValidityFilter',
             'DMARCReportInspectionFilter = afew.filters.DMARCReportInspectionFilter:DMARCReportInspectionFilter',
             'FolderNameFilter = afew.filters.FolderNameFilter:FolderNameFilter',
             'HeaderMatchingFilter = afew.filters.HeaderMatchingFilter:HeaderMatchingFilter',