Add test case verifying no t= (timestamp) with x= (expiration) in signature

dkim/tests/data/rfc6376.signed.no_t.msg

@@ -0,0 +1,20 @@
+DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; 
+ d=football.example.com; i=@football.example.com; 
+ q=dns/txt; s=test; h=from : to : subject : 
+ date : message-id : from : subject : date; x=100000000000; 
+ bh=4bLNXImK9drULnmePzZNEBleUanJCX5PIsDIFoH4KTQ=; 
+ b=icKcLSEZYXJ95flvWE8FT6hl5iqd8MC/LEKYH0QjsqYy6MO/4pgVNCZH
+ l/RAXAuADxE/40Fg7uTlxwwD1hjN2Ple6J//cJfslBdDOq6zTVbne1dqtl
+ NOat7iamJ1AfRqyG+ja7a2AZsrpUuJ7VA6O+0zRYPqpwMEkEFIzI9i/Xk=
+From: Joe SixPack <joe@football.example.com>
+To: Suzie Q <suzie@shopping.example.net>
+Subject: Is dinner ready?
+Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
+Message-ID: <20030712040037.46341.5F8J@football.example.com>
+
+Hi.
+
+We lost the game.  Are you hungry yet?
+
+Joe.
+

dkim/tests/test_dkim.py

@@ -62,6 +62,7 @@ class TestSignAndVerify(unittest.TestCase):
         self.message5 = read_test_data("rfc6376.signed.rsa.msg")
         self.message6 = read_test_data("test.message.baddomain")
         self.message7 = read_test_data("rfc6376.w1258.msg")
+        self.message8 = read_test_data("rfc6376.signed.no_t.msg")
         self.key = read_test_data("test.private")
         self.rfckey = read_test_data("rfc8032_7_1.key")
 
@@ -250,6 +251,7 @@ b/mPfjC0QJTocVBq6Za/PlzfV+Py92VaCak19F4WrbVTK5Gg5tW220MCAwEAAQ=="""
                 d = dkim.DKIM(self.message4)
                 res = d.verify(dnsfunc=self.dnsfunc5)
                 self.assertTrue(res)
+
     def test_non_utf8(self):
         # A message with Windows-1258 encoding is signed and verifies.
         for header_algo in (b"simple", b"relaxed"):
@@ -262,6 +264,13 @@ b/mPfjC0QJTocVBq6Za/PlzfV+Py92VaCak19F4WrbVTK5Gg5tW220MCAwEAAQ=="""
                 # As of 1.1.0 this won't verify, but at least we don't crash.  FIXME
                 self.assertFalse(res)
 
+    def test_no_t(self):
+        # Signature Timestamp is optional, so don't crash if it's missing.
+        d = dkim.DKIM(self.message8)
+        res = d.verify(dnsfunc=self.dnsfunc5)
+        # Signature won't verify, but that's not what we are testing.
+        self.assertFalse(res)
+
     def test_catch_bad_key(self):
         # Raise correct error for defective public key.
         d = dkim.DKIM(self.message5)