  Bit commitment (BC) is an important cryptographic primitive for an agent to
convince a mutually mistrustful party that she has already made a binding
choice of 0 or 1 but only to reveal her choice at a later time. Ideally, a BC
protocol should be simple, reliable, easy to implement using existing
technologies, and most importantly unconditionally secure in the sense that its
security is based on an information-theoretic proof rather than computational
complexity assumption or the existence of a trustworthy arbitrator. Here we
report such a provably secure scheme involving only one-way classical
communications whose unconditional security is based on no superluminal
signaling (NSS). Our scheme is inspired by the earlier works by Kent, who
proposed two impractical relativistic protocols whose unconditional securities
are yet to be established as well as several provably unconditionally secure
protocols which rely on both quantum mechanics and NSS. Our scheme is
conceptually simple and shows for the first time that quantum communication is
not needed to achieve unconditional security for BC. Moreover, with purely
classical communications, our scheme is practical and easy to implement with
existing telecom technologies. This completes the cycle of study of
unconditionally secure bit commitment based on known physical laws.
