Anonymous Key Distribution

   Jacek Cichon, Lukasz Krzywiecki, Miroslaw Kutylowski, Pawel Wlaz

We consider distribution of encryption keys in pay-per-view systems. The
communication  from  the  sender  to  the users is done through a single
broadcast channel. The protection mechanism is based upon encrypting the
broadcast  with  a  symmetric  key that has been distributed only to the
users that are currently ``logged in'' (that is, those  who  are  paying
for the information transmitted).


In such systems the major problem is updating the key when  the  set  of
users changes. We pursue two goals: communication related to a change of
the encryption key should be kept as small as possible. The second  goal
is  unrevealing  information about users behavior. Most systems proposed
so far disregard this issue and allow tracing preferences of  the  users
by somebody who is also logged in.

We present a procedure for distributing a new key in environments  where
the  set  of  users  changes  rapidly.  Our  scheme is based on balanced
allocation algorithms. The scheme is  simple,  easy  to  implement,  and
provides both anonymity and small communication overhead. An improvement
is achieved for practical parameter size.