Presently, we use an hrac as a bare identifying instrument in a few places. For example, the URL which Ursula exposes is /kFrag/{hrac_as_hex}. Let's make is /kFrag/{arrangement.id} per above. Both Alice and Bob will be able to calculate the arrangement.id, but it will prevent Sybil / Eve from poking various Ursulas in an effort to either see which Ursulas have a given KFrag or determine the value of n.