Cryptography – Particular algorithmic function encoding – Nbs/des algorithm
Patent
1997-01-30
1999-09-14
Cain, David
Cryptography
Particular algorithmic function encoding
Nbs/des algorithm
380 9, 380 49, H04L 900
Patent
active
059534214
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to a communications system using quantum cryptography.
As described in the present applicant's earlier applications Nos. PCT/GB93/02637 and PCT/GB93/02075, quantum cryptography is a technique for the distribution of a key for use in the subsequent encryption or decryption of transmissions in a communications system. The key is distributed by modulation of a single-photon signal. Any eavesdropper intercepting the single-photon signal disturbs the statistics of the received data, and so can be detected. The term "single-photon signal" encompasses any signal having the required quantum properties. It may be a true single-photon generated, for example, by parametric down-conversion. Such a technique is described and claimed in our earlier above-cited applications. Such true single-photon sources have been found however to be difficult to construct and run reliably. In general therefore it is preferable to take a "classical" source, such as a laser diode (LED) and to attenuate severely the output from such a source to a level such that in a given time slot there is in general no more than one, and on average very much less than one photon.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of communication using quantum cryptography including the steps of modulating a single-photon signal from a highly attenuated source, and detecting the modulated signal, characterised in that in the step of detecting the modulated signal the signal is split between two branches according to its encoded state, and signals corresponding to different encoded states are detected independently, and in that the rate of detection of coincident signals in the two branches of the detector is determined and compared with a threshold to detect the presence of an eavesdropper.
The present invention uses properties specific to signals from an attenuated source, and the eavesdropping techniques necessary to attack a signal from such a source, to provide a new approach to the detection of any eavesdropper. This enables the method of the present invention to provide enhanced security for the quantum cryptographic techniques previously proposed by the present applicants and others, and can also make possible the use of certain simplified encoding techniques for the quantum signal which, without the approach of the present invention, would not offer adequate security.
When a highly attenuated source is used, the attenuation is set at levels sufficient to make it highly unlikely that more than one photon would be present in any given time slot. To achieve this the average number of photons per time slot has to be considerably less than one. For example, the average level n may be 0.1.
An eavesdropper attempting to intercept the encoded key data might use a beam splitter to split off a fraction of the photon power while leaving the remainder to be transmitted on to the receiver. This however would in general lead to a marked reduction in the detection rate at the receiver unless the splitting ratio was very small, in which case the amount of useful data which the eavesdropper could extract would be very limited. In practice all real channels suffer from finite loss and this opens the possibility of a beamsplitting attack in which the eavesdropper compensates for her beamsplitting loss by substituting a lower loss channel. However, optical fibres already achieve the theoretical minimum loss, making it technically very difficult to substitute a channel of lesser loss, especially for channels of km lengths or greater. The eavesdropper might alternatively use an intercept/resend attack. As the name suggests, this involves the eavesdropper detecting destructively incoming signals and then re-sending an appropriate modulated signal for detection at the receiver. However, when the original intensity level is significantly less than 1, then retransmitting photon signals at that same intensity level in response to every detection at the ea
REFERENCES:
patent: 4965856 (1990-10-01), Swanic
patent: 5191614 (1993-03-01), LeCong
patent: 5243649 (1993-09-01), Franson
patent: 5307410 (1994-04-01), Bennett
patent: 5311592 (1994-05-01), Udd
patent: 5339182 (1994-08-01), Kimble et al.
patent: 5418905 (1995-05-01), Rarity et al.
patent: 5515438 (1996-05-01), Bennett et al.
patent: 5675648 (1997-10-01), Townsend
patent: 5757912 (1998-05-01), Blow
patent: 5764765 (1998-06-01), Phoenix et al.
Townsend et al, "Single Photon Interference in 10km Long Optical Fibre Interferometer", Electronics Letters, 29 (1993) Apr. 1, No. 7, Stevenage, Herts., GB.
Muller et al, "Experimental Demonstration of Quantum Cryptography Using Polarized Photons in Optical Fibre Over More than 1km", Europhysics Letters, 23 (6), pp. 383-388 (1993).
British Telecommunications public limited company
Cain David
LandOfFree
Quantum cryptography does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Quantum cryptography, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Quantum cryptography will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1517876