Basis reconciliation

Basis reconciliation is the classical post-processing step in quantum key distribution protocols such as BB84 in which the communicating parties publicly compare their basis choices and discard the bits for which their bases differed. Only the bits measured in matching bases are retained to form the sifted key.

The procedure is simple: Alice announces her sequence of basis choices over the public channel, Bob compares them to his own, and both parties keep only the bits where the choices matched. This step typically reduces the raw key by approximately half, since the probability of basis agreement is 50% when both parties choose randomly between two conjugate bases.

Basis reconciliation is not a security procedure. It is a sifting procedure. The security of the protocol comes from the fact that an eavesdropper who measured in the wrong basis during transmission has already introduced errors that will be detected in the subsequent error estimation phase. The authenticated classical channel used for basis reconciliation must itself be secure, or a man-in-the-middle attack could compromise the entire protocol.