Improving the boneh-franklin traitor tracing scheme

Traitor tracing schemes are cryptographically secure broadcast methods that allow identification of conspirators: if a pirate key is generated by k traitors out of a static set of l legitimate users, then all traitors can be identified given the pirate key. In this paper we address three practicality and security issues of the Boneh-Franklin traitor- tracing scheme. In the first place, without changing the original scheme, we modify its tracing procedure in the non-black-box model such that it allows identification of k traitors in time O˜(k2), as opposed to the original tracing complexity O˜(l). This new tracing procedure works independently of the nature of the Reed-Solomon code used to watermark private keys. As a consequence, in applications with billions of users it takes just a few minutes on a common desktop computer to identify large collusions. Secondly, we exhibit the lack of practical value of list- decoding algorithms to identify more than k traitors. Finally, we show that 2k traitors can derive the keys of all legitimate users and we propose afixtothis security issue. © International Association for Cryptologic Research 2009.