Secure Internet Exams Despite Coercion

TL;DR

This paper introduces new properties and a protocol to ensure coercion-resistance and anonymity in online exams, with formal proofs and improvements to existing mixnet security.

Contribution

It proposes two novel properties for coercion resistance, a new protocol satisfying these properties, and improves the security of the underlying mixnet used in secure exams.

Findings

The protocol is proven coercion-resistant in ProVerif.

A new attack and fix for the mixnet used in secure exams.

Enhanced anonymity properties for online exam protocols.

Abstract

We study coercion-resistance for online exams. We propose two properties, Anonymous Submission and Single-Blindness which, if hold, preserve the anonymity of the links between tests, test takers, and examiners even when the parties coerce one another into revealing secrets. The properties are relevant: not even Remark!, a secure exam protocol that satisfied anonymous marking and anonymous examiners results to be coercion resistant. Then, we propose a coercion-resistance protocol which satisfies, in addition to known anonymity properties, the two novel properties we have introduced. We prove our claims formally in ProVerif. The paper has also another contribution: it describes an attack (and a fix) to an exponentiation mixnet that Remark! uses to ensure unlinkability. We use the secure version of the mixnet in our new protocol.