  We propose a new Quantum Key Distribution method in which Alice sends pairs
of qubits to Bob, each in one of four possible states. Bob uses one qubit to
generate a secure key and the other to generate an auxiliary key. For each pair
he randomly decides which qubit to use for which key. The auxiliary key has to
be added to Bob's secure key in order to match Alice's secure key. This scheme
provides an additional layer of security over the standard BB84 protocol.
