The Trouble with Quantum Bit Commitment

TL;DR

This paper provides the first complete proof that the 1993 BCJL quantum bit commitment protocol is insecure, clarifying previous misconceptions and demonstrating its vulnerability through a subtle attack.

Contribution

It offers the first comprehensive proof of the BCJL protocol's insecurity, correcting earlier claims and refining understanding of quantum bit commitment vulnerabilities.

Findings

The BCJL protocol is insecure against quantum attacks.

Previous claims of security for BCJL were incomplete.

A subtle attack successfully breaks the BCJL protocol.

Abstract

In a recent paper, Lo and Chau explain how to break a family of quantum bit commitment schemes, and they claim that their attack applies to the 1993 protocol of Brassard, Cr\'epeau, Jozsa and Langlois (BCJL). The intuition behind their attack is correct, and indeed they expose a weakness common to all proposals of a certain kind, but the BCJL protocol does not fall in this category. Nevertheless, it is true that the BCJL protocol is insecure, but the required attack and proof are more subtle. Here we provide the first complete proof that the BCJL protocol is insecure.