Efficient traitor tracing for clone attack in content protection

In this paper we design efficient traitor tracing scheme for a pirate clone attack against a broadcast-encryption-based content protection system. In this content protection system, each user(device) is assigned a set of secret keys. In a clone attack, pirates (legitimate users) compromise their devices, extract their secret keys and use those keys to build a clone device. The clone device allows decryption of content that is originally only accessible by legitimate devices. The pirates can sell the clone device for profits. When a clone device is recovered, a traitor tracing scheme could identify which compromised devices' (called traitors) keys are in the clone. Once the compromised keys are detected, they can be disabled for future content access. In the process of tracing traitors, a series of carefully constructed cipher text is fed into the clone device and the reaction of the clone device is observed and used to deduce which keys are contained inside the clone. The traceability of a tracing scheme is measured by the number of testing cipher texts needed to identify the traitors. The state-of-art traitor tracing schemes in the symmetric key setting achieve O(t³ log t) traceabilities for t traitors. Unfortunately the theoretically efficient polynomial traceability could convert to years' tracing time in reality. In this paper, we present a practical approach that combines traditional traitor tracing scheme design with system security engineering consideration by introducing a "software key conversion data" virtual program. This combination enables our approach to drastically improve traceability over the state-of-art traitor tracing scheme existed in applied cryptography community. The traceabilities for clone attack is improved from O(t³ log t) to O(t) which converts the tracing time from the original 15 years to 4 hours for a clone attack of 100 traitors. Our much improved traceabilities makes them ultimately adopted to use in AACS [1], the new industry content protection standard for next generation high definition DVDs.