Co-Sign: A Lightweight, Peer-to-Peer Protocol for Cryptographic Proof of Co-Location

This paper introduces "Co-Sign," a lightweight and efficient peer-to-peer cryptographic protocol that generates a secure proof of co-location, addressing the challenge of physical interaction verification which is often vulnerable to location spoofing. By leveraging a TEE-agnostic challenge-response flow and a collabora-tive signing mechanism, Co-Sign enables two or more parties to securely create a proof of meeting without requiring specialized hardware. We present the formal protocol design, a security analysis, a functional reference implementation in Python, and a performance evaluation that demonstrates its efficiency for real-world applications requiring high accountability.