Premature basis announcement in quantum key distribution

If basis information and test positions are announced before the receiver confirms receipt of quantum carriers, an eavesdropper can store the carriers until learning that information. In a scheme sending one half of each Bell pair after a Hadamard gate encoding, the eavesdropper then forwards test qubits untouched and measures all data qubits in their now-known correct bases. Every test passes, while data are changed to classically correlated product states whose bit labels the eavesdropper knows. This invalidates the sampling argument needed to justify entanglement purification and quantum key distribution. A sound additional purification certification could detect this attack and abort; it cannot generate private Bell pairs from the separable attacked data. Announcing bases only after receipt prevents this particular delayed-carrier attack.