Key Distribution versus Key Enhancement in Quantum Cryptography

It has been said that quantum cryptography in general o ers a secure solution to the problem of key enhancement. This means that two parties who already share a small secret key, can use quantum protocols to establish a new large secret key. This large secret key can be arbitrarily long and is unbreakable. Thus, to date, the main contribution of quantum cryptography has been believed to be quantum key enhancement. This paper shows that quantum cryptography can do signi cantly more. The quantum protocol described here distributes an unbreakable secret key to the two parties by relying on public information only. This is the rst time that quantum cryptography is shown to be able to produce secret information using only public information. This contribution is also unique for cryptography in general, classical and quantum.