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Privacy amplification compresses a partially secret random string into a shorter key by a publicly selected hash function, reducing an adversary's information about the final key. In the BB84 protocol it follows parameter estimation and error correction, and accounts for information leaked in the public reconciliation transcript. Authentication of the public channel remains necessary.

Quantum key distribution uses a quantum channel and an authenticated public classical channel to establish matching random keys for two parties while limiting an eavesdropper's information. The public transcript is included in the adversary's information. In an ideal successful protocol, a key of length $\ell$ is uniform and independent of that information. Actual protocols allow small disagreement and secrecy errors and may abort. Measurement in quantum mechanics disturbance provides parameter tests, while privacy amplification removes residual knowledge after classical reconciliation. The BB84 protocol and entanglement-based protocols are examples.