Secure multiparty computation with verifiable real-world locations and times
I Prove, Therefore I Am: Spatiotemporal Multi-Party Computation
Cryptography and Security
Summary
When multiple people want to work together on sensitive data but don’t trust each other, they can use special computer methods to keep their information private. This paper explores how to include real-world facts like where and when each person is during the computation, without revealing those details or letting anyone lie about them. The authors develop a way to prove someone really was at a specific place and time by completing certain checks, and they show how to use this idea to build secure systems that combine private data and physical presence. Their work also looks at how these proofs hold up even against attackers who use quantum computers.
What this means in practice
- •For blockchain developers: Integrate verifiable proofs of location and time into decentralized applications to enhance trust in physical-world conditions linked to transactions.$Commercial implications: Enables location- and time-based smart contracts that can be sold to industries requiring physical event verification, such as supply chain or real estate.
- •For secure cloud service teams: Provide cloud computation services guaranteeing clients’ inputs reflect real physical states without revealing sensitive location or timing details.
Authors
Ziqing Guo, Fuyuki Kitagawa, Xiao Liang
Abstract
Secure multiparty computation (MPC) enables mutually distrustful parties to compute on private digital inputs. We initiate the study of spatiotemporal MPC, extending this paradigm to functionalities whose inputs additionally depend on physical facts such as the parties' locations, times, or trajectories. Such protocols must simultaneously hide spatiotemporal information and ensure its physical consistency: a malicious party should not be able to make the functionality operate on a spatiotemporal input inconsistent with its actual physical state. The main conceptual challenge is to formulate extraction of spatiotemporal information within the simulation-based security framework. We introduce arguments of spatiotemporal knowledge, following the principle "I prove, therefore I am:" rather than defining physical presence directly through a mathematical relation, we define it operationally through the ability to complete a sound spatiotemporal verification protocol. Accordingly, an extractor recovers a spatiotemporal point from a successful prover and certifies its physical validity by using the extracted prover to succeed in an auxiliary spatiotemporal verification protocol. Building on this notion, we define universally composable (UC) security for spatiotemporal MPC, capturing privacy, physical consistency, and composability. We provide constructions achieving this new MPC notion. We first construct UC-secure commit-and-prove of spatiotemporal knowledge: in the CRS model under LWE against quantum provers without pre-shared entanglement, and in the QROM against quantum provers with unbounded pre-shared entanglement. Using these protocols, we obtain UC-secure spatiotemporal MPC from semi-honest post-quantum MPC. We also extend our framework to support quantum functionalities with classical spatiotemporal input.