Symmetric Tardos fingerprinting codes for arbitrary alphabet sizes

Fingerprinting provides a means of tracing unauthorized redistribution of digital data by individually marking each authorized copy with a personalized serial number. In order to prevent a group of users from collectively escaping identi cation, collusion-secure ngerprinting codes have been proposed. In this paper, we introduce a new construction of a collusion-secure ngerprinting code which is similar to a recent construction by Tardos but achieves shorter code lengths and allows for codes over arbitrary alphabets. For binary alphabets, n users and a false accusation probability of , a code length of m c0 ln(n= ) is provably suÆcient to withstand collusion attacks of at most c0 colluders. This improves Tardos' construction by a factor of 10. Furthermore, invoking the Central Limit Theorem we show that even a code length of m 1 2 2c20 ln(n= ) is suÆcient in most cases. For q-ary alphabets, assuming the restricted digit model, the code size can be further reduced. Numerical results show that a reduction of 35% is achievable for q = 3 and 80% for q = 10.