vPass: Publicly Verifiable Fair Exchange Protocol for Vehicle Passports

Abstract

In second-hand vehicle markets, blockchains are being proposed as means to provide verification of vehicle history, a.k.a. vehicle passport (VP). However, given that confidentiality of VPs often contradicts public verification, blockchains are not used to their full potential in the proposed frameworks. Specifically, although blockchain smart contracts offer a decentralized mechanism for untrusted parties to fairly exchange digital assets without the need for a trusted third party, VP exchange is always carried off-chain. In this work, we investigate the problem of “fair exchange” of confidential VPs over public blockchains where its plain information must be verified against its publicly committed value. We propose a zero-knowledge proof, called Consistent Commitment Encryption (CCE), that enables the public verification of the consistency between ElGamal encryption of a given VP and its Pedersen commitment. We employ our CCE to build vPass, a decentralized vehicle passport framework that enables second-hand vehicle buyers to purchase vehicle history information from designated service providers and get it verified and delivered on-chain while preserving its confidentiality. The security of CCE relies on the intractability of the discrete logarithm problem in elliptic curve groups and it has no trusted setup. We formally prove that CCE is sound, complete, and witness indistinguishable proof of knowledge, and report on comparisons with other generic proof systems. Moreover, we show that vPass provides fair exchange and confidentiality of the vehicle history, and compare it to existing VP systems. Finally, we provide a proof of concept implementation on Ethereum and report the system performance metrics.