  We propose a new classical bit commitment protocol using the relativistic
constraint that signals cannot travel faster than the speed of light $c$. This
protocol is unconditionally secure against both classical or quantum attacks.
The sender (Alice) and the receiver (Bob) each controls two secure stations
separated by a large distance $d$, and they communicate by exchanging classical
information only. Alice commits by sending from her stations two causally
independent encrypted messages to the neighboring Bob's stations, after that
the protocol is out of her control and she plays no role in the unveiling
phase. The commitment remains concealed for a period of $\Delta t=d/2c$. This
protocol requires only limited communication resources and is readily
implementable with current technologies.
