hsfz: Improve alive check dissection. (#4760)

Author: antoniovazquezblanco

scapy/contrib/automotive/bmw/hsfz.py

@@ -65,12 +65,23 @@ class HSFZ(Packet):
         ConditionalField(
             StrFixedLenField("identification_string",
                              None, None, lambda p: p.length),
-            lambda p: p.control == 0x11 and p.length != 0)
+            lambda p: p._hasidstring())
     ]
 
     def _hasaddrs(self):
         # type: () -> bool
-        return self.control == 0x01 or self.control == 0x02
+        # Address present in diagnostic_req_res, acknowledge_transfer and
+        # two byte length alive_check frames.
+        return self.control == 0x01 or \
+            self.control == 0x02 or \
+            (self.control == 0x12 and self.length == 2)
+
+    def _hasidstring(self):
+        # type: () -> bool
+        # ID string is present in some vehicle_ident_data frames and in
+        # long alive_check grames.
+        return (self.control == 0x11 and self.length != 0) or \
+            (self.control == 0x12 and self.length > 2)
 
     def hashret(self):
         # type: () -> bytes

test/contrib/automotive/bmw/hsfz.uts

@@ -111,9 +111,21 @@ assert pkt.length == 0
 assert pkt.control == 0x11
 
 
-= Test HSFZSocket
+= Dissect alive check
+pkt = HSFZ(bytes.fromhex("000000200012444941474144523130424d5756494e5858585858585858585858585858585858"))
+assert pkt.length == 32
+assert pkt.control == 0x12
+assert b"BMW" in pkt.identification_string
+
+pkt = HSFZ(bytes.fromhex("00000002001200f4"))
+assert pkt.length == 2
+assert pkt.control == 0x12
+assert pkt.source == 0x00
+assert pkt.target == 0xf4
 
 
+= Test HSFZSocket
+
 server_up = threading.Event()
 def server():
     buffer = bytes(HSFZ(control=1, source=0xf4, target=0x10) / Raw(b'\x11\x22\x33' * 1024))