CA-less Mutual Co-Signing of Documents over a Unidirectional Visual Channel with Transported Hardware Attestation

Abstract

We describe and analyze a protocol for mutual co-signing of a document by two mobile devices that (i) communicate only over a one-way, lossy, low-bandwidth optical channel (an animated on-screen code read by the counterparty's camera), (ii) use no intermediary server on the trust path, and (iii) use no certificate authority. Trust in each party's public key is instead grounded in a hardware attestation token produced by the platform secure element, transported in full over the visual channel by a rateless (fountain) code and cryptographically bound into the co-signature. The core technical contribution is a two-stage hash anchor that removes the circular signing dependency inherent to interactive co-signing: the first party commits to the document before the identity of the second party is known, and the second party's identity is later bound to that commitment without invalidating the first signature. We give a threat model, define four security properties (anchor binding, co-signature inseparability, attestation-bound key provenance, and post-signing tamper evidence) and reduce them to standard assumptions (collision resistance of H and EUF-CMA security of the underlying signature scheme), with the secure element modeled as an ideal signing oracle. We report a working instantiation on iOS/Android using ECDSA P-256 in the Secure Enclave/StrongBox, SHA-256, Apple App Attest / Play Integrity, and an LT-style fountain code, together with an independent third-party verifier that recomputes all anchors and checks both signatures fully offline.