Catch binascii related key format errors (LP: #1854477)

dkim/__init__.py

@@ -37,6 +37,7 @@ import hashlib
 import logging
 import re
 import time
+import binascii
 
 # only needed for arc
 try:
@@ -688,6 +689,9 @@ class DomainSigner(object):
     except KeyFormatError as e:
       self.logger.error("%s" % e)
       return False
+    except binascii.Error as e:
+      self.logger.error('KeyFormatError: {0}'.format(e))
+      return False
 
     # RFC 8460 MAY ignore signatures without tlsrpt Service Type
     if self.tlsrpt == 'strict' and not self.seqtlsrpt: