A Revocation Scheme Preserving Privacy

           Lukasz Krzywiecki, Przemyslaw Kubiak, Miroslaw Kutylowski

We introduce a new scheme for anonymous user exclusion in an encrypted
broadcast communication. It allows a broadcaster to change the transmission key
with a single message sent to N users through a broadcast channel, so that all 
but z excluded  users can retrieve the new key, and volume of the message
changing the key is proportional to z.

Our scheme is based on Shamir's secret sharing method, but we propose
to use polynomials with dynamic coefficients and shares that evolve in time. 
Such a modification hides information on which users are excluded
and when, thus ensures privacy.