  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.
