  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.
