2022-9: "Zero-Knowledge Age Restriction for GNU Taler"
We propose a design for a privacy-friendly method of age restriction in e-commerce that is aligned with the principle of subsidiarity. The design is presented as an extension of a privacy-friendly payment protocol with a zero-knowledge scheme that cryprographically augments coins for this purpose. Our scheme enables buyers to prove to be of sufficient age for a particular transaction without disclosing it. Our modification preserves the privacy and security properties of the payment system such as the anonymity of minors as buyers as well as unlinkability of transactions. We show how our scheme can be instantiated with ECDSA as well with a variant of EdDSA, respectively, and how it can be integrated with the GNU Taler payment system. We provide formal proofs and implementation of our proposal. Key performance measurements for various CPU architectures and implementations are presented.