Validate UUID portion of IDProperty

stix2/properties.py

@@ -102,9 +102,12 @@ class IDProperty(Property):
         super(IDProperty, self).__init__()
 
     def validate(self, value):
-        # TODO: validate GUID as well
         if not value.startswith(self.required_prefix):
             raise ValueError("must start with '{0}'.".format(self.required_prefix))
+        try:
+            uuid.UUID(value.split('--', 1)[1], version=4)
+        except Exception:
+            raise ValueError("must have a valid version 4 UUID after the prefix.")
         return value
 
     def default(self):

stix2/test/test_properties.py

@@ -63,8 +63,13 @@ def test_id_property():
     idprop = IDProperty('my-type')
 
     assert idprop.validate('my-type--90aaca8a-1110-5d32-956d-ac2f34a1bd8c')
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError) as excinfo:
         idprop.validate('not-my-type--90aaca8a-1110-5d32-956d-ac2f34a1bd8c')
+    assert str(excinfo.value) == "must start with 'my-type--'."
+    with pytest.raises(ValueError) as excinfo:
+        idprop.validate('my-type--foo')
+    assert str(excinfo.value) == "must have a valid version 4 UUID after the prefix."
+
     assert idprop.validate(idprop.default())