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Charles H. Bennett and Gilles Brassard proposed quantum key distribution using qubits prepared in two incompatible measurement bases. The sender and receiver disclose their basis choices and retain events with matching bases; they test a sample for disturbance before deriving a key. A simple intercept-and-resend attack illustrates the principle: an eavesdropper choosing a random one of the two bases chooses incorrectly half the time and then causes a wrong retained bit half the time, giving a $1/4$ error rate. This example is an attack analysis, rather than a complete security proof.^[ref]

Gilles Brassard is a computer scientist whose work with Charles H. Bennett established an influential approach to quantum key distribution. Their 1984 protocol, now called BB84, encodes information in pure states represented by nonorthogonal vectors and uses classical communication to compare the preparation and measurement choices.^[ref]

The physical idea is that an observer who does not know the preparation basis cannot extract the encoded information without risking detectable disturbance. The protocol therefore uses sampled outcomes to assess the shared data. Its classical discussion must be authenticated: otherwise an active intermediary could impersonate each participant. This connects the physics of quantum measurement with assumptions about the surrounding cryptographic protocol.^[ref]