Quantum contract signing with entangled pairs

TL;DR

This paper introduces a quantum protocol for contract signing using entangled states that ensures fairness, privacy, and security based on physical laws, allowing one client to sign independently and minimizing trust in a third party.

Contribution

The protocol is the first to enable a single client to sign contracts independently without the other, and it relies solely on quantum physics principles rather than classical cryptographic assumptions.

Findings

Protocol achieves arbitrarily small cheating probability scaling as N^{-1/2}

Clients can sign contracts independently without mutual presence

The scheme is abuse-free and based on physical laws

Abstract

We present a quantum scheme for signing contracts between two clients (Alice and Bob) using entangled states and the services of a third trusted party (Trent). The trusted party is only contacted for the initialization of the protocol, and possibly at the end, to verify clients' honesty and deliver signed certificates. The protocol is {\em fair}, i.e., the probability that a client, say Bob, can obtain a signed copy of the contract, while Alice cannot, can be made arbitrarily small, and scales as $N^{-1/2}$, where $4N$ is the total number of rounds (communications between the two clients) of the protocol. Thus, the protocol is {\em optimistic}, as the cheating is not successful, and the clients rarely have to contact Trent to confirm their honesty by delivering the actual signed certificates of the contract. Unlike the previous protocol [Paunkovi\'c, et al., 2017], in the present proposal, a single client can obtain the signed contract alone, without the need for the other client's presence. When first contacting Trent, the clients do not have to agree upon a definitive contract. Moreover, even upon terminating the protocol, the clients do not reveal the actual contract to Trent. Finally, the protocol is based on the laws of physics, rather than on mathematical conjectures and the exchange of a large number of signed authenticated messages during the actual contract signing process. Therefore, it is {\em abuse-free}, as Alice and Bob cannot prove they are involved in the contract signing process.