One-out-of-many proofs: Or how to leak a secret and spend a coin

Abstract

We construct a 3-move public coin special honest verifier zero-knowledge proof, a so-called Sigma-protocol, for a list of commitments having at least one commitment that opens to 0. It is not required for the prover to know openings of the other commitments. The proof system is efficient, in particular in terms of communication requiring only the transmission of a logarithmic number of commitments. We use our proof system to instantiate both ring signatures and zero coin, a novel mechanism for bitcoin privacy. We use our Sigma-protocol as a (linkable) ad-hoc group identification scheme where the users have public keys that are commitments and demonstrate knowledge of an opening for one of the commitments to unlinkably identify themselves (once) as belonging to the group. Applying the Fiat-Shamir transform on the group identification scheme gives rise to ring signatures, applying it to the linkable group identification scheme gives rise to zero coin. Our ring signatures are very small compared to other ring signature schemes and we only assume the users’ secret keys to be the discrete logarithms of single group elements so the setup is quite realistic. Similarly, compared with the original zero coin protocol we only rely on a weak cryptographic assumption and do not require a trusted setup. A third application of our Sigma protocol is an efficient proof of membership of a secret committed value belonging to a public list of values.