  The Goldenberg-Vaidman (GV) protocol for quantum key distribution (QKD) uses
orthogonal encoding states of a particle. Its security arises because
operations accessible to Eve are insufficient to distinguish the two states
encoding the secret bit. We propose a two-particle cryptographic protocol for
quantum secure direct communication, wherein orthogonal states encode the
secret, and security arises from restricting Eve from accessing any
two-particle operations. However, there is a non-trivial difference between the
two cases. While the encoding states are perfectly indistinguishable in GV,
they are partially distinguishable in the bi-partite case, leading to a
qualitatively different kind of information-vs-disturbance trade-off and also
options for Eve in the two cases.
