International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Zulfikar Ramzan

Publications

Year
Venue
Title
2006
CRYPTO
2006
CRYPTO
2006
PKC
2006
EPRINT
Fast Algorithms for the Free Riders Problem in Broadcast Encryption
Zulfikar Ramzan David P. Woodruff
We provide algorithms to solve the free riders problem in broadcast encryption. In this problem, the broadcast server is allowed to choose some small subset F of the revoked set R of users to allow to decrypt the broadcast, despite having been revoked. This may allow the server to significantly reduce network traffic while only allowing a small set of non-privileged users to decrypt the broadcast. Although there are worst-case instances of broadcast encryption schemes where the free riders problem is difficult to solve (or even approximate), we show that for many specific broadcast encryption schemes, there are efficient algorithms. In particular, for the complete subtree method and some other schemes in the subset-cover framework, we show how to find the optimal assignment of free riders in O(|R||F|) time, which is independent of the total number of users. We also define an approximate version of this problem, and study specific distributions of R for which this relaxation yields even faster algorithms. Along the way we develop the first approximation algorithms for the following problem: given two integer sequences a_1 >= a_2>= ... >= a_n and b_1 >= b_2 >= ... >= b_n, output for all i, an integer j' for which a_{j'} + b_{i-j'} <= (1+\epsilon) min_j a_j + b_{i-j}. We show that if the differences a_i - a_{i+1}, b_i-b_{i+1} are bounded, then there is an O(n^{4/3}/\epsilon^{2/3})-time algorithm for this problem, improving upon the O(n^2) time of the naive algorithm.
2005
ASIACRYPT
2004
ASIACRYPT
2004
PKC
2000
CRYPTO
1999
CRYPTO
1999
FSE

Program Committees

Crypto 2008
Crypto 2007