Paper 2009/196

Analysis of one quantum bit string commitment

Zhengjun Cao

Abstract

A. Kent proposed a quantum bit string commitment protocol in 2003. Not using the standard two conjugate states $|0\rangle$, $|1\rangle $, $|+\rangle $ and $|-\rangle $, the protocol uses $\psi_0=|0\rangle$ and $\psi_1=\sin \theta|0\rangle+\cos \theta |1\rangle $, where $\theta\neq 0$. In this paper, we show that the protocol can not guarantee security to the receiver. $(1-\frac{\sin^2 \theta}{2})n $ bits are definitely exposed to the receiver, where $n$ is the length of the committed string.