Traitor-tracing systems allow identifying the users who contributed to building a rogue decoder in a broadcast environment. In a traditional traitor-tracing system, a key authority is responsible for generating the global public parameters and issuing secret keys to users. All security is lost if the key authority itself is corrupt. This raises the question: Can we construct a traitor-tracing scheme, without a trusted authority?

In this work, we propose a new model for traitor-tracing systems where, instead of having a key authority, users could generate and register their own public keys. The public parameters are computed by aggregating all user public keys. Crucially, the aggregation process is public, thus eliminating the need of any trusted authority. We present a new traitor-tracing system in this model based on bilinear pairings. Our scheme is proven adaptively secure in the generic group model. This scheme features a transparent setup, ciphertexts consisting of $6\sqrt{L}+4$ group elements, and a public tracing algorithm. Our main technical ingredient is a new registered functional encryption (RFE) scheme for quadratic functions. This is joint work with Russell W. F. Lai, Monosij Maitra, Giulio Malavolta, Ahmadreza Rahimi and Ivy K. Y. Woo.