Robust ambiguity for contact tracing

TL;DR

This paper introduces a modified contact tracing protocol that enhances privacy by preventing malicious users from compromising the system, addressing a key vulnerability in existing decentralized contact tracing methods.

Contribution

It presents a robust, malicious-user-resistant protocol for contact tracing that improves privacy protections over previous DH-based approaches.

Findings

The protocol achieves robustness against malicious users.

It maintains privacy properties in contact tracing.

Security properties are formally established.

Abstract

A known drawback of `decentralised' contact tracing architectures is that users who have been in contact with an infected person are able to precisely identify the relevant contact, and thereby perhaps identify the infected person. In their proposal, the PACT team discuss a simple DH-based protocol to mitigate this problem, but dismiss it because it is vulnerable to a malicious user who may deviate from the specified behaviour. This note presents a modified protocol which achieves robustness against a fully malicious user, and establishes some simple security properties.