Necessity of parameter randomization in quantum contract signing

Abstract

We present a proof that randomization is necessaryin quantum contract signing protocol of Paunković, Bauda andMateus. We prove that for any fixed value of the protocolparameter $\alpha$, for large $N$ the probability of cheating canbe as high as $25\mathrm{\%}$, where $N$ is the number of messagesexchanged between the parties, and thus without randomization theprotocol is not fair.