Study of Broadcast Encryption and New Approaches Towards It.

Date of Submission

December 2009

Date of Award

Winter 12-12-2010

Institute Name (Publisher)

Indian Statistical Institute

Document Type

Master's Dissertation

Degree Name

Master of Technology

Subject Name

Computer Science


Applied Statistics Unit (ASU-Kolkata)


Sarkar, Palash (ASU-Kolkata; ISI)

Abstract (Summary of the Work)

This report is a study of Broadcast Encryption (BE) schemes primarily concentrating on the widely used Subset Cover Framework. We start with the basic framework of Broadcast Encryption, defining the various terminologies that are found in the BE literature. The study of schemes concentrates primarily on the Complete Subtree and the Subset Difference schemes. We also study the more recent work, the Punctured Interval scheme. We summarize the results of the significant schemes at the end of our study.As our contribution, we propose two new frameworks: the Hitting Set Framework and the Interval Framework. We describe the Hitting Set Framework in which finding the minimal set of keys for a transmission can be mapped to the bipartite matching problem which has well known polynomial time solutions. Then we describe the Interval Framework and propose two new schemes in this framework. The first scheme, L-DAG gives results similar to the Punctured Interval scheme (N − 1 keys per user for N users for the header size of r + 1 where there are r revoked users). The other I-L-DAG scheme achieves much better results (log N keys per user for the header size of r) but at the cost of resilience. We also propose a new tradeoff between keys per user and the header size using our first scheme L-DAG in which, increasing the header size to r + k − 1 from r, we can decrease the number of keys by a factor of k making it 2N/k − 1. We compare our scheme with the Punctured Interval scheme, and suggest some improvements to the latter.Then we propose an improvisation on the tree-based schemes: the k-ary tree scheme based on the subset-cover framework. In this, we combine the ideas used in the Complete Subtree scheme and the Subset Difference scheme to perform the key pre-distribution with the help of k-ary trees where k ≥ 3. This helps us achieve a reduced number of keys per user while the header size grows to 3r − 2. As an example, we start with the Ternary Tree scheme from which we develop the idea for the more general k-ary tree scheme. Further, in the k-ary tree scheme, for a fixed N, we study the variation in the number of keys stored per user by varrying k. We argue that there will be a k for which the number of keys stored per user will be minimmum


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Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.


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