The most successful application of quantum theory to cryptography is quantum key distribution (QKD). The goal of QKD is to generate an identical string of bits that is privately shared between two parties, which we shall call Alice and Bob.
The particular QKD scheme that we will describe was proposed by Charles Bennett and Gilles Brassard in 1984, and is often referred to as BB84.
The protocol has two main parts, a quantum and classical phase. In the quantum phase, Alice sends single photons to Bob over some public quantum channel. In the classical phase, Alice and Bob need to talk to each other over an authenticated classical channel, that is, it can be public but they need to verify that they are talking to the correct person.
