Making it easier to get an hrac from outside PolicyGroup.

nkms/policy/models.py

@@ -110,6 +110,14 @@ class PolicyGroup(object):
                 pass  # Tell Alice to either wait or lower the value of n.
 
     def hrac(self):
+        """
+        A convenience method for generating an hrac for this instance.
+        """
+        return self.hrac_for(self.alice, self.bob, self.uri)
+
+    @staticmethod
+    def hrac_for(alice, bob, uri):
+
         """
         The "hashed resource authentication code".
 
@@ -121,7 +129,7 @@ class PolicyGroup(object):
         Alice and Bob have all the information they need to construct this.
         Ursula does not, so we share it with her.
         """
-        return self.hash(bytes(self.alice.seal) + bytes(self.bob.seal) + self.uri)
+        return PolicyGroup.hash(bytes(alice.seal) + bytes(bob.seal) + uri)
 
     def treasure_map_dht_key(self):
         """