UNCONDITIONALLY SECURE QUANTUM BIT COMMITMENT IS IMPOSSIBLE

The claim of quantum cryptography has always been that it can provide protocols that are unconditionally secure, that is, for which the secu rity does not depend on any restriction on the time, space, or technol ogy available to the cheaters. We show that this claim does not hold F or any quantum bit commitment protocol. Since many cryptographic tasks use bit commitment as a basic primitive, this result implies a severe setback for quantum cryptography. The model used encompasses all reas onable implementations of quantum bit commitment protocols in which th e participants have not met before, including those that make use of t he theory of special relativity.