Generalizing Multi-party Contract Signing

TL;DR

This paper introduces DAG MPCS protocols that combine linear and parallel execution, enabling flexible, hierarchical multi-party contract signing with proven fairness and efficiency bounds.

Contribution

It presents a new class of DAG-based protocols for multi-party contract signing, with formal semantics, fairness criteria, and complexity bounds, extending prior linear and parallel approaches.

Findings

DAG MPCS protocols can model hierarchical signer structures.

Fairness is characterized by absence of abort-chaining sequences.

Lower bounds on complexity measures for fair protocols are established.

Abstract

Multi-party contract signing (MPCS) protocols allow a group of signers to exchange signatures on a predefined contract. Previous approaches considered either completely linear protocols or fully parallel broadcasting protocols. We introduce the new class of DAG MPCS protocols which combines parallel and linear execution and allows for parallelism even within a signer role. This generalization is useful in practical applications where the set of signers has a hierarchical structure, such as chaining of service level agreements and subcontracting. Our novel DAG MPCS protocols are represented by directed acyclic graphs and equipped with a labeled transition system semantics. We define the notion of abort-chaining sequences and prove that a DAG MPCS protocol satisfies fairness if and only if it does not have an abort-chaining sequence. We exhibit several examples of optimistic fair DAG MPCS protocols. The fairness of these protocols follows from our theory and has additionally been verified with our automated tool. We define two complexity measures for DAG MPCS protocols, related to execution time and total number of messages exchanged. We prove lower bounds for fair DAG MPCS protocols in terms of these measures.