  We suggest that the randomness of the choices of measurement basis by Alice
and Bob provides an additional important resource for quantum cryptography. As
a specific application, we present a novel protocol for quantum key
distribution (QKD) which enhances the BB84 scheme by encrypting the information
sent over the classical channel during key sifting. We show that, in the limit
of long keys, this process prevents an eavesdropper from reproducing the
sifting process carried out by the legitimate users. The inability of the
eavesdropper to sift the information gathered by tapping the quantum channel
reduces the amount of information that an eavesdropper can gain on the sifted
key. We further show that the protocol proposed is self sustaining, and thus
allows the growing of a secret key.
