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 $|0⟩$, $|1⟩$, $|+⟩$ and $|-⟩$, the protocol uses ${\psi }_0=|0⟩$ and ${\psi }_1=\sin \theta |0⟩+\cos \theta |1⟩$, where $\theta \ne 0$. 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 $$ is the length of the committed string.