A tree is used to partition stateless receivers in a broadcast content
encryption system into subsets. Two different methods of partitioning are
disclosed. When a set of revoked receivers is identified, the revoked
receivers define a relatively small cover of the non-revoked receivers by
disjoint subsets. Subset keys associated with the subsets are then used
to encrypt a session key that in turn is used to encrypt the broadcast
content. Only non-revoked receivers can decrypt the session key and,
hence, the content.