Quantum cryptography has several unorthodox attributes: it is invulnerable to passive eavesdropping; communicators need no initial shared secret (cryptographic key), but they do need an auxiliary tamper-proof link; the scheme requires an uninterrupted light path (no repeater), and a one-time pad of keystream must be prepared in advance of the secure transmission. At least two other cryptologic schemes share these same attributes. This is quite remarkable because each of the three schemes has an entirely different physical basis for its message secrecy. In quantum cryptography an eavesdropper cannot measure or clone the state of a photon without revealing the attempt to the authorized receiver. The second scheme is the Yuen–Kim protocol. Potential bits for the keystream are masked by classical noise. The eavesdropper cannot extract the same useful bits that the authorized receiver extracts because their receivers are statistically independent. Our own scheme, called QDRN, distributes broadband noisy light to terminals, where interferometers provide identical keystreams. Security presumes that there exists some bandwidth broad enough so that the eavesdropper cannot store the phase information either optically or digitally for some period like minutes, or even hours if necessary, after which the users may safely transmit data. The Yuen–Kim protocol is by far the simplest to implement. However, it is limited to point-to-point links and distances of some tens of kilometers. By contrast, QDRN operates with full power, is compatible with amplifiers and networks, and extends to hundreds of kilometers, quite possibly a megameter.
PACS: 03.67.Dd