Private Yoking Proofs: Attacks, Models and New Provable Constructions

We present two attacks on the security of the private grouping proof by Batina et al. [1]. We introduce the first formal models for yoking proofs. One model incorporates the aspect time, ensuring that the grouping proofs were generated at a specific time. A more general variant only provides a proof that tags were together at some time. Based on these models we propose two new protocols to generate sound yoking proofs that can trivially be extended to multiple parties and that attain narrow-strong privacy.