Cryptographic receiver

Details Cryptographic receiver Cryptographic receiver

1996-04-10

2000-02-22

Hayes, Gail O.

Cryptography

Particular algorithmic function encoding

Nbs/des algorithm

380 44, 380 49, H04L 912

Patent

active

06028935&

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a cryptographic receiver, and more particularly to a cryptographic receiver for a communications system in which a cryptographic key-code is derived by two parties as a result of an exchange of information between the two parties following measurement of individual photons.
2. Discussion of Prior Art
Cryptographic receivers incorporated within cryptographic communications systems have been described previously, for instance in International Application No. PCT/GB91/01352 with a Publication No. of WO92/04785, and by A. K. Ekert et al, in Physical Review Letters, vol 69, number 9, August 1992, pages 1293 to 1295. The communications systems described therein comprise two cryptographic receivers separated by some distance and a source of correlated photon pairs. The receivers each include a filter whose transmission characteristics are modulated in response to a respective control signal. Exchange of information about the control signals between the two receivers enables each one to derive a common key-code, available to the receivers only. This key-code enables further messages to be encrypted and transmitted using insecure communications links. These systems have the disadvantage that high speed modulators are required which have associated high transmission losses.
A cryptographic communications system based on a photon polarization measuring receiver has been described by A. Muller et al, in European Physics Letters, vol 23, 1993 pages 383 to 388. The receiver described therein comprises an electro-optic switch which switches photons between two channels, one of which contains a 45.degree. rotator. The channels recombine and photon polarizations are separated by a polarization beam splitter before being detected.


SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alternative form of cryptographic receiver.
The present invention provides a cryptographic receiver for receiving individual information carrying photons comprising: photon channel and the second photon channel; basis on photons within the first photon channel; basis on photons within the second photon channel, the first and second measurement bases being non-orthogonal; output signals upon detection of such photons; and from an external source and for generating a cryptographic key-code from a combination of said output signals and said signals from an external source such that the receiver and the external source share an identical key-code, the first and second photon channels without the requirement for a control signal, and the respective measurement basis imposed on each received photon.
The invention provides the advantage that a high speed modulator is not required, high speed modulators having associated high transmission losses. Another advantage is that transmission rates may be higher than the prior art receivers.
The cryptographic receiver may be arranged to receive photons where the information is carried in the form of a phase relationship. The receiver may include unbalanced Mach-Zehnder interferometers whose transmission characteristics are dependent on the photon wavelength. The cryptographic receiver may be arranged alternatively to measure the polarization of received photons, the polarization state carrying the information.
The cryptographic receiver of the invention may be incorporated within a communications system having a second cryptographic receiver of the invention, each receiver providing a common key-code to a respective party. This arrangement provides the advantage of enhanced security since neither party provides a control signal for their receiver, such signals being vulnerable to interception.


BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
FIG. 1 schematically illustrates a cryptographic receiver of the invention;
FIG. 2 schematically illustrates a cryptographic receiver of the inventi

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Bennett, Charles H., "Quantum Cryptography Using Any Two Nonorthogonal States", Physical Review Letters, vol. 68, No. 21, May 25, 1992, pp. 3121-3124.
Townsend et al., "Enhanced Single Photon Fringe Visibility in a 10 km-Long Prototype Quantum Cryptography Channel", Electronics Letters, vol. 29, No. 14, Jul. 8, 1993, pp. 1291-1293.

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