WO2010151105A1

WO2010151105A1 - Method for use in quantum cryptography

Info

Publication number: WO2010151105A1
Application number: PCT/MY2010/000107
Authority: WO
Inventors: Sellami Ali; Mohamed Ridza Wahiddin
Original assignee: Mimos Berhad
Priority date: 2009-06-24
Filing date: 2010-06-23
Publication date: 2010-12-29
Also published as: MY149272A

Abstract

A method for use in quantum cryptography system, particularly for enhancing the security level of obtaining the cryptographic key to encode the information communicated between to legitimate parties. A plurality of reference signals (RS) with different frequencies and amplitudes are transmitted by Alice to Bob, followed by transmitting a plurality of quantum signals (QS) with different interval times and amplitudes. Bob measures the arrival time of the reference signals (RS) and quantum signals (QS) at his end, and reports to Alice. Alice provides the accurate arrival times and Bob discards the erroneous ones. Based on the preferred embodiments of the present invention, the key is obtained by way of measuring the the arrival times of reference signals (RS) and quantum signals (QS).

Description

METHOD FOR USE IN QUANTUM CRYPTOGRAPHY

FIELD OF INVENTION

The present invention relates generally to a method for use in quantum cryptography and more particularly to a method for enhancing and thus allowing a more secured transmission of cryptographic keys of a quantum key distribution system.

BACKGROUND OF INVENTION

Current methods and systems for ensuring secured communication or sharing information between two parties, a sender ("Alice") and a receiver ("Bob") in an open communication channel in absolute secrecy involve the effectiveness in delivering the cryptographic keys to both communicating parties. In this process, weak signals based on the cryptographic key are sent or transmitted over a quantum channel. It is a well established fact that the cryptographic key allows the recipient to decrypt delivered information or messages.

A common adversity in such quantum communication approach is the presence of an eavesdropper known as "Eve", often introducing errors to the keys. This however is considered as an unavoidable disturbance and eavesdropping will definitely be revealed to the legitimate communicating parties. This unique property of being alerted on the presence of the eavesdropper and that their communication has been compromised is a unique property of quantum cryptography communication systems. However, it is discovered that "Eve" can adapt to current technologies to enhance or devise her measurement capabilities in determining or gaining knowledge of the key.

The revolutionary changes in technology over the past decades have resulted to the development of diverse techniques and protocols configured particularly to avoid disturbances by eavesdropper, Eve and thus enhance the security level or threshold for producing a cryptographic key. Most of the current implementations to confuse Eve on measuring the key and heightened security are based on, among others, variation in measuring arrival time of signals or pulses between sender and receiver also known as time frequency uncertainty, Alice and Bob respectively, polarization-based coding, phase coding and issuing decoy signals within the channel.

An exemplary of a current approach or implementation in quantum cryptography involves the sender Alice sends photons to the receiver, Bob and these photons may be sent based on different states and orientations, which are also known as polarizations. Bob, being the receiver therefore must measure the photons he receives based on his selected states and orientations. Alice aids to solve this and thus provide the key for decryption by way providing the accurate states and orientations to Bob.

In the above case of BB84 and many other protocols, it is disovered that Eve can exploit multi-photon pulses in a lossy line to perform the photon-number-splitting attack whereby she would count the photons in each pulse, and whenever this number is larger than one, she keeps one photon in a quantum memory and forwards the remaining photons to Bob on a lossless line. To protect and thereby mask the transmission from that kind of attack, Alice sends randomly the quantum signals (QS) with different amplitude. One of the quantum signal (QS) pulses is used to distribute the key and the second one as decoy signal pulse is used to detect Eve. In this case Eve can not distinguish between the quantum signals as long as the photon numbers of the pulses are the same.

In implementations where measurement is merely performed based on the time as the fundamental factor, Eve may be able to obtain a perfect copy of the key after the reconciliation process, whereby Eve only has to send back to Bob a photon pulse within a duration much lesser than the total time thereby resulting to confusing Bob instead due to incorrect measurement of pulses at his end.

Following the above, although many methods may be expedient in ensuring secured delivery of cryptographic keys within the quantum key distribution system, the main challenge which would normally surface in most cases, is the need of continuous alignment of the system, particularly for polarization-based systems. It is known that in polarization- based systems the polarization has to be maintained stable typically over tens of kilometers in order to keep aligned the polarizers at Alice's and Bob's sides. To maintain the alignment can be an encumbrance to many users.

Similarly, for systems emphasizing interferometric approach, the system involves two unbalanced Mach-Zechnder interferometers, wherein at least one interferometer is adjusted to alternate with a second interferometer for every few seconds so as to compensate thermal drifts.

The present invention therefore seeks to provide an efficient scheme and method based on time and frequency factors which is expected to solve the aforementioned problems and primarily avoiding attacks by Eve. This proposed method is used for security and forcing Eve to make errors and thus protect the quantum channel.

Accordingly, it is a primary object of the present invention to provide method for use in enhancing the security of transmission of cryptographic key within a quantum channel.

It is yet another object of the present invention to provide a method for use in enhancing the security of a quantum key distribution (QKD) system that is not affected by polarization transformation in standard single mode communication fibers, random and changing with time.

It is a further object of the present invention to provide a method for use in enhancing security of a quantum key distribution (QKD) system, which can eliminate the drawback in phase drift in interferometer.

It is yet another object of the present invention to provide a method for use in enhancing security of a quantum key distribution (QKD) system wherein it is most suitable for use in fiber based and free space QKD systems and quantum network.

It is yet a further object of the present invention to provide a method for use in enhancing security of a quantum key distribution (QKD) system that is formed to be robust against propagation medium disturbances. It is yet another object of the present invention to provide a method for use in quantum key distribution (QKD) system that can be implemented with minimum amount of devices.

Further objects and advantages of the present invention may become apparent upon referring to the preferred embodiments of the present invention as shown in the accompanying drawings and as described in the following description.

SUMMARY OF INVENTION

The present invention provides a method for use in quantum cryptography comprising the steps of: transmitting a plurality of reference signals (RS) with different frequencies and amplitudes within a predetermined period (T); transmitting a plurality of quantum signals (QS) with different interval times and amplitudes within a predetermined period

(t_/-n); said period is less than that of the period (T) for transmitting reference signals (RS); receiving the plurality of reference signals (RS) and quantum signals (QS); measuring the arrival times for said reference signals (RS) and quantum signals (QS); providing the accurate arrival time measurements for both reference signals (RS) and quantum signals (QS); discarding the inaccurate arrival time measurements for both reference signals (RS) and quantum signals (QS); providing the accurate basis of reference signals (RS) frequencies; checking the frequencies of the reference signals (RS) based on the provided accurate basis of reference signals (RS) frequencies; obtaining the cryptographic keys based on the gained accurate basis of reference signals (RS) frequencies and accurate arrival times of both reference signals (RS) and quantum signals (QS). BRIEF DESCRIPTION OF DRAWINGS

Other objects, features, and advantages of the invention will be apparent from the following description when read with reference to the accompanying drawings. In the drawings, wherein like reference numerals denote corresponding parts throughout the several views:

FIG 1 shows the transmission of reference signals (RS) in different frequencies and amplitudes followed by transmission of quantum signals (QS) in different interval times and amplitudes in accordance with a preferred embodiment of the present invention; and

DETAILED DESRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those or ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures and/or components have not been described in detail so as not to obscure the invention. Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

Essentially, the method of the present invention is developed based on the uncertainty in information between time and frequency. Accordingly, there are two photon sources, these are quantum signal source (QS) and reference signal source (RS). Based on these photon sources, the cryptographic key is obtained by way of measuring the arrival time between the reference signal (RS) and quantum signal (QS) at Bob's end. These reference signals (RS) are preferably sent with differing frequencies and amplitude within a predetermined period of time (T) so as to heighten the level of uncertainty or ambiguity particularly for Eve.

The Debuisschert and Boucher protocol is used as a reference with respect to the present invention, in which this particular approach emphasizes coding ambiguous and non ambiguous detection. The method of the present invention however allows the measurement of pulses in separated in time, for instance the delay of quantum signal (QS) and reference signal (RS) is preferably used for coding. As in the monitoring line, the method uses an approach of sending pulses randomly with different frequency, whilst sending pulses with different amplitudes aids to prevent photon number splitting attack (PNS).

FIG 1 shows the steps of Alice sending a reference signal (RS) with different frequencies and amplitudes, whereby said reference signal is used or taken as timing reference. Subsequently, Alice sends a quantum signal (QS) which is intended for preparing the cryptographic key to Bob. It is noted that the sending time of quantum signals with different amplitude is randomly prepared and based on t_n, where trn<tn<trn+l .

Bob therefore measures the time of arrival for the reference and quantum signals, and accordingly reports to Alice on the results of measurement, as in the arrival time of the ^" reference and quantum signals at his end. Alice then accordingly informs Bob on which are accurate. Based on Alice's information, Bob keeps the accurate basis for obtaining the key and discards the erroneous ones. This process can be seen in FIG 2. In the final step, Bob reveals the items that he has detected in the data line. This step is known as the reconciliation step, which therefore results to the sifted or filtration of the key. Alice and Bob then evaluate the quantum bit error rate (denoted as Q) which is the scarifying part of the sifted key. In order to share the key, two additional standard steps are performed, said steps will be described herein shortly.

The first step in obtaining and thus share the key is to remove the errors from the key based on an error correction algorithm. Next, cancellation is performed with respect to the information of Eve on the key, said cancellation is based on a privacy amplification algorithm.

As briefly described in the preceding paragraphs, the information is coded by means of the delay between reference signals (RS) and quantum signals (QS). Alice sends the reference pulses with different frequencies thereby giving the time reference. In order to encode the key, an additional delay with respect to that reference can be put on each quantum signals (QS). It is noted that preferred delays chosen are smaller than the pulse duration of reference signal (RS).

From here, Bob uses photocounters with a time resolution much better than the pulse duration of the reference signal. He then evaluates the delay by way of measuring the detection time with respect to the reference. It is noted that the measurement of pulses can be performed well separated in time. As mentioned previously, if a measurement is only performed in the time domain, Eve would be able to obtain a perfect copy of the key after the reconciliation process. In order to do this, Eve will only need to send back to Bob one photon pulses with a duration 7E much smaller than T and with a delay identical to the one she measured. Bob cannot distinguish T pulses from TE pulses with only one measurement.

Therefore, the method of the present invention is able to prevent such attack by way of sending reference signals (RS) with different frequency randomly where Eve would not be able to distinguish the various frequencies of reference signals. Accordingly, Eve is forced to make errors and thus the quantum channel can be protected. In parallel to the measurement in the time domain, Bob performs a measurement in the frequency domain.

Bob sends at random the pulses he receives to a frequency meter to measure reference signal pulses (RS) frequency. Bob and Alice evaluate the measurement of frequencies state by exchanging basis via public channel between them. As briefly mentioned earlier, Bob keeps the right basis of the reference signals (RS) with frequencies f0 and fl and make them as reference time for quantum signals (QS) and to be used to obtain the cryptographic key. The other arm of the input beam splitter is sent to the photocounter that is used to establish the key between Alice and Bob. The control of the reference signals (RS) via frequency measurements allows coding the information only in the time domain.

Based on the preferred embodiments of the present invention, the photocounters can have a response time lesser than 1 nanoseconds (ns). The preferred pulse duration is within 10-20 nanoseconds (ns) range.

We will thus consider pulse durations in the 10-20 ns range for which the time propagation of the pulses is only little affected by the propagation disturbances of the fiber. It should be noted that a low error rate requires precisions in the arrival time of about 1 nanosecond (ns) which therefore makes it insensitive to fiber thermal dilatation. The pulse spreading due to group velocity dispersion starts to be noticeable only in the ps range with the application of usual telecommunication fibers. The measurement of the arrival time of the photon does not require that the polarization of the photon be conserved. The measurement method of frequency is insensitive to polarization; in other words the whole system incorporated with the method of the present invention is potentially insensitive to polarization.

As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its essential characteristics. The present embodiments is, therefore, to be considered as merely illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within therefore intended to be embraced therein.

Claims

A method for use in quantum cryptography comprising the steps of:

transmitting a plurality of reference signals (RS) with different frequencies and amplitudes within a predetermined period (T); transmitting a plurality of quantum signals (QS) with different interval times and amplitudes within a predetermined period (t_/-n); said period is less than that of the period (T) for transmitting reference signals (RS); receiving the plurality of reference signals (RS) and quantum signals (QS); measuring the arrival times for said reference signals (RS) and quantum signals (QS); providing the accurate arrival time measurements for both reference signals (RS) and quantum signals (QS); discarding the inaccurate arrival time measurements for both reference signals (RS) and quantum signals (QS); providing the accurate basis of reference signals (RS) frequencies; checking the frequencies of the reference signals (RS) based on the provided accurate basis of reference signals (RS) frequencies; obtaining the cryptographic keys based on the gained accurate basis of reference signals (RS) frequencies and accurate arrival times of both reference signals (RS) and quantum signals (QS).

2. The method as claimed in Claim 1 further comprising the steps of removing the errors from the potential cryptographic keys based on an error algorithm and Danceling any information of an eavesdropper in relation to the key.

3. The method as claimed in Claims 1 and 2 wherein the information to be communication through the quantum channel is coded based on delays between the transmitted reference signals (RS) and quantum signals (QS).

4. The method as claimed in Claims 1 to 3 wherein the period of delays are preferably less than that of the pulse duration of the reference signals (RS).

5. The method as claimed in Claims 1 to 4 wherein obtaining the key further comprising the step of providing an additional delay with respect to the reference signals (RS) on the quantum signals (QS).

6. The method as claimed in Claims 1 to 5 wherein checking the frequencies of the received reference signals (RS) by exchanging frequency basis between the sender and receiver via a public channel.

7. The method as claimed in Claims 1 to 6 wherein the arrival time of the reference signals (RS) is controlled by way of shifting the frequency of reference signals randomly.

8. The method as claimed in Claims 1 to 7 wherein the reference signals (RS) are sent with different frequencies randomly.