Paper 2025/2253

Efficient Privacy-Preserving Blueprints for Threshold Comparison

Abstract

Privacy-Preserving Blueprints (PPBs), introduced by Kohlweiss et al. in in EUROCRYPT 2023, offer a method for balancing user privacy and bad-actor detection in private cryptocurrencies. A PPB scheme allows a user to append a verifiable escrow to their transactions which reveals some identifying information to an authority in the case that the user misbehaved. A natural PPB functionality is for escrows to reveal user information if the user sends an amount of currency over a certain threshold. However, prior works constructing PPBs for such a functionality have severe limitations when it comes to efficiency: escrows are either computationally infeasible to compute, or too large to be plausibly stored on a large-scale distributed ledger. We address these gaps by constructing a space and computation-efficient PPB for threshold comparison, producing escrows under 2kb that can be computed in seconds. The scheme can be instantiated using well-known cryptographic primitives, namely variants of the ElGamal encryption scheme and generic non-interactive zero-knowledge proofs. As an additional contribution, we implement one of the theoretical generic PPB constructions originally proposed by Kohlweiss et al. and find that it performs surprisingly well in practice. For the threshold comparison functionality it requires approximately 14kb escrows, and can be computed in around 12 seconds.