Cryptography – Particular algorithmic function encoding – Nbs/des algorithm
Patent
1996-04-22
1998-06-09
Cangialosi, Salvatore
Cryptography
Particular algorithmic function encoding
Nbs/des algorithm
380 59, H04L 908
Patent
active
057647658
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to a system for the communication of encrypted data using quantum cryptography.
Quantum cryptography is a method for distributing a secret key between users of a communications system in such a way that the security of the key can be guaranteed. This is done by testing for variations in the statistics of transmitted data which occur when an eavesdropper intercepts signals on a quantum channel. Examples of such techniques are disclosed in C. H. Bennett, F. Bessette, G. Brassard, L. Salvail and J. Smolin, "Experimental Quantum Cryptography", Journal of Cryptology, 5 3 (1992) and in the other references cited below. Recent work by the present applicants in this field is described and claimed in our co-pending international applications PCT/GB93/02075 and PCT/GB93/02637 and in two further international applications filed this day entitled "QUANTUM CRYPTOGRAPHY ON A MULTIPLE ACCESS NETWORK" (U.S. application Ser. No. 08/605,048, pending) and "QUANTUM CRYPTOGRAPHY" (U.S. application Ser. No. 08/612,880, pending). The disclosures of these co-pending applications are incorporated herein by reference.
Hitherto, all communications systems using quantum cryptography have required that at least one of the two parties establishing the secret key should have a source of single-photon signals and/or a single-photon detector. The original proposals for quantum cryptographic systems all described point-to-point links between two users (conventionally named Alice and Bob). Our above-cited international applications disclose a method whereby this basic technique may be extended to multiple-access systems in which different respective secret keys are established between the transmitter Alice and a number of receivers Bob (1) . . . Bob(N).
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of communication using quantum cryptography characterised in that two stations each independently modulate a single-photon signal transmitted from an external source to both of the stations in series, the said signal subsequently passing on to a third station which detects the state of the signal, compares it with the state of the signal as originally transmitted, and communicates the results of the comparison to the two stations, the two stations thereby establishing a shared secret key.
The present invention provides a new method of quantum cryptography which allows any pair of users connected, for example, to an optical network, to establish a secret key between them. This key is private to the two users and is known neither to any other stations connected to the network, nor to the transmitter which provides the source of the single-photon signals. The users Bob(1) and Bob(2) need only modulate the single-photon signal and so do not need to have the expensive and specialised equipment necessary to produce or detect single-photons. This is a significant practical advantage by contrast with previous proposals which have required that at least one of the two parties establishing the secret key should have the means to produce and/or detect single photons.
Preferably the third station includes the source for the single-photon signal and transmits the signal from the source encoded in a predetermined phase or polarisation state, and includes a single-photon detector and measures the state of the single-photon signal as returned from the two stations.
Preferably a multiplicity of stations (Bob(1), Bob(2) . . . Bob(N)) are connected to the transmitter station by a common communications network, and the method further comprises an initial contention step in which a selected two of the multiplicity of stations establish an exclusive right to use specific transmissions of the single-photon signal for establishing a mutual secret key.
The present invention may be used, for example, with an optical-fibre based LAN, such as the well known FDDI optical fibre ethernet network. With such a network there might be, for example, ten client s
REFERENCES:
patent: 5243649 (1993-09-01), Franson
patent: 5307410 (1994-04-01), Bennett
patent: 5339182 (1994-08-01), Kimble et al.
patent: 5418905 (1995-05-01), Rarity et al.
patent: 5515438 (1996-05-01), Bennett et al.
Barnett Stephen Mark
Phoenix Simon James
British Telecommunications public limited company
Cangialosi Salvatore
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