A combinatorial approach to asymmetric traitor tracing

Abstract

To protect against illegal copying and distribution of digital objects, such as images, videos and software products, merchants can ‘fingerprint’ objects by embedding a distinct codeword in each copy of the object, hence allowing unique identication of the buyer. The buyer does not know where the codeword is embedded and so cannot tamper with it. However a group of dishonest buyers can compare their copies of the object, nd some of the embedded bits and change them to create a pirate copy. A c-traceability scheme can identify at least one of the colluders if up to c colluders have generated a pirate copy. In this paper we assume the merchant is not trusted and may attempt to ‘frame’ a buyer by embedding the buyer’s codeword in a second copy of the object. We introduce a third party called the ‘arbiter’ who is trusted and can arbitrate between the buyer and the merchant if a dispute occurs. We describe the system as a set system and give two constructions, one based on polynomials over nite elds and the other based on or-thogonal arrays, that provide protection in the above scenario.