  In counterfactual quantum key distribution (QKD), two remote parties can
securely share random polarization-encoded bits through the blocking rather
than the transmission of particles. We propose a semi-counterfactual QKD, i.e.,
one where the secret bit is shared, and also encoded, based on the blocking or
non-blocking of a particle. The scheme is thus semi-counterfactual and not
based on polarization encoding. As with other counterfactual schemes and the
Goldenberg-Vaidman protocol, but unlike BB84, the encoding states are
orthogonal and security arises ultimately from single-particle non-locality.
Unlike any of them, however, the secret bit generated is maximally
indeterminate until the joint action of Alice and Bob. We prove the general
security of the protocol, and study the most general photon-number-preserving
incoherent attack in detail.
