Information-Theoretic Description of No-go Theorem of a Bit Commitment

We give a comprehensive and constructive proof of the no-go theorem of a bit commitment given by Mayers, Lo, and Chau from the viewpoint of quantum information theory. It is shown that there is a trade-off relation between information acquired by Bob during the commitment phase and the ability to change a commit bit by Alice during the opening phase. It is clarified that a protocol that is unbiased to both Alice and Bob cannot be, at the same time, secure against both parties. Fundamental physical constraints that govern this no-go theorem are also discussed.