In BB84, the key is generated using the quantum states of photons. The sender, traditionally named Alice, sends a sequence of qubits to the receiver, Bob. Each qubit is prepared in one of four states: horizontal, vertical, or diagonal. Bob measures each qubit using randomly chosen bases. After the transmission, Alice and Bob compare their bases over a classical channel and retain the bits where their bases matched, discarding the rest. This process results in a shared key.
