Optimizing Segment Based Document Protection^*

Abstract

We consider documents with restricted access rights, where some segments of the document are encrypted in order to prevent unauthorized reading. The access rights to such a document are described by an access graph. It is a directed acyclic graph; each node describing a different access rights level. It is assumed that a user having the rights corresponding to a node v has also all rights corresponding to all nodes w such that there is a directed path from v to w in the access graph. Then, to each node v we assign a key K_v and use this key to encrypt the segment of the document corresponding to the access level v.
We consider key management schemes and encoding auxiliary information in the document which ensure that a user who gets a single key corresponding to his access level v can derive all keys K_w for w = v or w being an ancestor of v in the access graph.
In this paper we show how to minimize the total size of auxiliary keying information stored in the document. We provide an algorithm based on node disjoint paths in the access graph and key derivation based on one-way functions. We show that the algorithm chooses the paths in an optimal way.

Published in Proceedings of 38th Conference on Current Trends in Theory and Practice of Computer Science (SOFSEM), Czech Republic, January 21-27, 2012, Lecture Notes in Computer Science, vol. 7147, Springer-Verlag 2012, pp. 566-575.

^*This research has been partially supported by Foundation for Polish Science, Programme "MISTRZ"

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