  When studying safety properties of (formal) protocol models, it is customary
to view the scheduler as an adversary: an entity trying to falsify the safety
property. We show that in the context of security protocols, and in particular
of anonymizing protocols, this gives the adversary too much power; for
instance, the contents of encrypted messages and internal computations by the
parties should be considered invisible to the adversary.
  We restrict the class of schedulers to a class of admissible schedulers which
better model adversarial behaviour. These admissible schedulers base their
decision solely on the past behaviour of the system that is visible to the
adversary.
  Using this, we propose a definition of anonymity: for all admissible
schedulers the identity of the users and the observations of the adversary are
independent stochastic variables. We also develop a proof technique for typical
cases that can be used to proof anonymity: a system is anonymous if it is
possible to `exchange' the behaviour of two users without the adversary
`noticing'.

Cryptography and Security