Broadcast Encryption with Free-Riders and Unconditional Security

We propose two schemes for efficient broadcast key establishment that enables a sender to communicate to any subset of the user-base by allowing a small ratio of free-riders. The schemes do not require stateful receivers and one scheme is unconditionally secure. The free-riders are unable to learn from the past whether they might become free-riders for a certain transmission again. We present a new trade-off facet for broadcast encryption, namely the number (or ratio) of free-riders vs. the number of messages to be sent or the number of keys stored by each user. Finally, our schemes follow an approach, related to the gradual exchange of a secret, which has been previously mainly considered in the context of fair exchange. As such, it offers a new approach to the problem of broadcast encryption, which may result in further advanced protocols.