Traitors collaborating in public: Pirates 2.0

This work introduces a new concept of attack against traitor tracing schemes. We call attacks of this type Pirates 2.0 attacks as they result from traitors collaborating together in a public way. In other words, traitors do not secretly collude but display part of their secret keys in a public place; pirate decoders are then built from this public information. The distinguishing property of Pirates 2.0 attacks is that traitors only contribute partial information about their secret key material which suffices to produce (possibly imperfect) pirate decoders while allowing them to remain anonymous. The side-effect is that traitors can publish their contributed information without the risk of being traced; giving such strong incentives to some of the legitimate users to become traitors allows coalitions to attain very large sizes that were deemed unrealistic in some previously considered models of coalitions. This paper proposes a generic model for this new threat, that we use to assess the security of some of the most famous traitor tracing schemes. We exhibit several Pirates 2.0 attacks against these schemes, providing new theoretical insights with respect to their security. We also describe practical attacks against various instances of these schemes. Eventually, we discuss possible variations on the Pirates 2.0 theme.