Upper bounds of a class of imperfect quantum sealing protocols

The model of the quantum protocols sealing a classical bit is studied. It is shown that there exist upper bounds on its security. For any protocol where the bit can be read correctly with the probability $\alpha$, and reading the bit can be detected with the probability $\beta$, the upper bounds are $\leqslant 1/2$ and $\alpha +\beta \leqslant 9/8$.Comment: 5 pages, 1 figure. Add an example on how to break the majority voting schem

Comment on "Unconditionally secure commitment in position-based quantum cryptography"

We show that a recently proposed relativistic commitment scheme [Sci. Rep. 4, 6774 (2014). arXiv:1406.6679] is insecure against the cheating of the committer.Comment: 1 pag

Unconditionally secure quantum coin flipping

Quantum coin flipping (QCF) is an essential primitive for quantum cryptography. Unconditionally secure strong QCF with an arbitrarily small bias was widely believed to be impossible. But basing on a problem which cannot be solved without quantum algorithm, here we propose such a QCF protocol, and show how it manages to evade all existing no-go proofs on QCF.Comment: The protocol is modified so that the security proof can be simplified. Also corrected a flaw in the analysis on the no-go proof in Ref.[13]. We thank the anonymous referee for pinpointing out the fla