Necessity of parameter randomization in quantum contract signing

Hana Almoner Louka

We present a proof that randomization is necessary in quantum contract signing protocol of Paunković, Bauda and Mateus. We prove that for any fixed value of the protocol parameter $\alpha$, for large $N$ the probability of cheating can be as high as $25\%$, where $N$ is the number of messages exchanged between the parties, and thus without randomization the protocol is not fair.