Quantum Bit Commitment and Coin Tossing Protocols

Hoi‐Kwong Lo, H. F. Chau

Hoi-Kwong Lo, H.F. Chau

Guanghui Liu, Dong Cao

Guanghui Liu, Dong Cao

Quantum coin tossing is a simple cryptographic primitive with many applications in more complicated tasks and plays a crucial role in complex quantum cryptography protocol design. Coin tossing protocols in classical environment are often based on bit commitment protocol. It has been proved that unconditionally secure quantum bit commitment schemes are insecure. Therefore, quantum coin tossing based on quantum bit commitment protocol isn't unconditionally secure. In general, quantum coin tossing is strictly weaker than bit commitment. In this paper, we have proposed a quantum coin tossing protocol based on quantum public-key cryptosystems. According to quantum CSS codes, we constructed a class of quantum quasi-cyclic low-density parity-check codes, and then proposed a novel quantum coin tossing protocol. Security analysis show that the proposed protocols are more secure.

Ariel Danan, Lev Vaidman A

Hoi‐Kwong Lo, H. F. Chau

There had been well known claims of ``provably unbreakable'' quantum protocols for bit commitment and coin tossing. However, we, and independently Mayers, showed that all proposed quantum bit commitment (and therefore coin tossing) schemes are, in principle, insecure because the sender, Alice, can always cheat successfully by using an EPR-type of attack and delaying her measurements. One might wonder if secure quantum bit commitment and coin tossing protocols exist at all. Here we prove that an EPR-type of attack by Alice will, in principle, break {\em any} realistic quantum bit commitment and {\em ideal} coin tossing scheme. Therefore, provided that Alice has a quantum computer and is capable of storing quantum signals for an arbitrary length of time, all those schemes are insecure. Since bit commitment and coin tossing are useful primitives for building up more sophisticated protocols such as zero-knowledge proofs, our results cast very serious doubt on the security of quantum cryptography in the so-called ``post-cold-war'' applications.