Correctly handle verification of signatures without t= (timestamp) and with x= (expiration); both are optional (LP: 2071892)

ChangeLog

@@ -1,4 +1,6 @@
 2024-07-04 Version 1.1.8
+   - Correctly handle verification of signatures without t= (timestamp) and
+     with x= (expiration); both are optional (LP: 2071892)
 
 2024-06-23 Version 1.1.7
    - Fix error in validate_signature_fields which prevented signature

dkim/__init__.py

@@ -331,6 +331,8 @@ def validate_signature_fields(sig, mandatory_fields=[b'v', b'a', b'b', b'bh', b'
         t_sign = int(sig[b't'])
         if t_sign > now + slop:
             raise ValidationError("t= value is in the future (%s)" % sig[b't'])
+    else:
+        t_sign = None
 
     if b'v' in sig and sig[b'v'] != b"1":
         raise ValidationError("v= value is not 1 (%s)" % sig[b'v'])
@@ -345,7 +347,7 @@ def validate_signature_fields(sig, mandatory_fields=[b'v', b'a', b'b', b'bh', b'
         if x_sign < now - slop:
             raise ValidationError(
                 "x= value is past (%s)" % sig[b'x'])
-        if x_sign < t_sign:
+        if t_sign and x_sign < t_sign:
             raise ValidationError(
                 "x= value is less than t= value (x=%s t=%s)" %
                 (sig[b'x'], sig[b't']))