A Broadcast Key Distribution Scheme Based on Block Designs

A key distribution scheme for broadcast encryption is proposed. It is based on block designs. In this scheme a centre provides each user (receiver) of the system with a set of keys. If at a later stage, some users are no longer entitled to the messages, they should also no longer be able to decrypt them. This should even be the case if these illegitimate users can form a coalition and exchange the keys that they have obtained before (provided that the size of this coalition does not exceed some value).