The self-blindable U-Prove scheme by Hanzlik and Kluczniak is forgeable, by Eric Verheul and Sietse Ringers and Jaap-Henk Hoepman

In "A Short Paper on How to Improve U-Prove Using Self-Blindable Certificates" by L. Hanzlik and K. Kluczniak (FC'2014), an unlinkable version of the U-Prove attribute-based credential scheme is proposed. Unfortunately, the new scheme is forgeable: if sufficiently many users work together then they can construct new credentials, containing any set of attributes of their choice, without any involvement of the issuer. In this short paper we show how they can achieve this and we point out the error in the unforgeability proof.