Optimizing Segment Based Document Protection

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.