Quantum cryptography is a method for secure key exchange over an insecure channel based on the nature of photons. Photons have a polarization, which can be measured in any basis, where a basis consists of two directions orthogonal to each other. If a photon's polarization is read in the same basis twice, the polarization will be read correctly and will remain unchanged. If it is read in two different bases, a random answer will be obtained in the second basis, and the polarization in the initial basis will be changed randomly.

The following protocol can be used by Alice and Bob to exchange secret keys.

Alice sends Bob a stream of photons, each with a random polarization, in a random basis. She records the polarizations.

Bob measures each photon in a randomly chosen basis and records the results.

Bob announces, over an authenticated but not necessarily private channel (e.g., by telephone), which basis he used for each photon.

Alice tells him which choices of bases are correct.