Bilkent University
Department of Computer Engineering


Optimal Broadcast Encryption with Free Riders


Murat Ak
Computer Engineering Department
Bilkent University

Broadcast encryption schemes allow a center to broadcast encrypted messages so that each particular message can only be decrypted by a set of privileged receivers designated for it. The standard technique is to distribute keys to all receivers at the beginning and to use only the necessary keys to encrypt the message for each particular broadcast. The number of encryptions needed constitutes the transmission cost in broadcast encryption schemes. The most efficient scheme in terms of transmission cost published so far is the subset difference (SD) scheme. It is possible to reduce the transmission overhead of a broadcast encryption scheme by allowing a number of free riders to be able to decrypt the message although they are not privileged. However, unless the free riders are chosen cleverly, the cost may even increase. In this thesis, we deal with the problem of choosing a given number of free riders effectively to reduce the transmission overhead in SD scheme. We first present three greedy algorithms. First algorithm has a fast execution, the second one is slower but has lower transmission overhead, and the last one has a trade-off between running time and transmission cost. Then we present two algorithms which find the minimum transmission overhead possible given a free rider quota. The experiments we conduct show that the transmission cost can be significantly reduced.


DATE: September 4, 2006, Monday@ 15:30