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Laser Physics

Robustness of quantum cryptography: SARG04 key-distribution protocol

Dmitry Anatolevich Kronberg, S. N. Molotkov

Published Apr 1, 2009• 11 citations
Publisher

Abstract

The SARG04 protocol is aimed at the improvement of the robustness of the main protocols in quantum cryptography (primarily, BB84 and B92) against the PNS attack in the case of the application of weak coherent pulses instead of the one-particle signals. An optimal attack of eavesdropper on the transmitted key is explicitly constructed for arbitrary angles between the basis states. A critical error up to which the key distribution is possible and the length of the final key are determined.

DOI
10.1134/s1054660x09040495

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•Oct 1, 2012•0 citations
The SARG04 protocol is aimed at the improvement of the robustness of the main protocols in quantum cryptography (primarily, BB84 and B92) against the PNS attack in the case of the application of weak coherent pulses instead of the one-particle signals. An optimal attack of eavesdropper on the transmitted key is explicitly constructed for arbitrary angles between the basis states. A critical error up to which the key distribution is possible and the length of the final key are determined.
The first quantum cryptography protocol, proposed by Bennett and Brassard in 1984 (BB84), has been widely studied in recent years. This protocol uses four states (more precisely, two complementary bases) for the encoding of the classical bit. Recently, it has been noticed that by using the same four states, but a different encoding of information, one can define a protocol which is more robust in practical implementations, specifically when attenuated laser pulses are used instead of single-photon sources [V. Scarani et al., Phys. Rev. Lett. 92, 057901 (2004), referred to as the SARG04 protocol]. We present a detailed study of SARG04 in two different regimes. In the first part, we consider an implementation with a single-photon source: we derive bounds on the error rate QQQ for security against all possible attacks by the eavesdropper. The lower and the upper bound obtained for SARG04 ($Q\ensuremath{\lesssim}10.95%$ and $Q\ensuremath{\gtrsim}14.9%$, respectively) are close to those obtained for BB84 ($Q\ensuremath{\lesssim}12.4%$ and $Q\ensuremath{\gtrsim}14.6%$, respectively). In the second part, we consider a realistic source consisting of an attenuated laser and improve on previous analysis by allowing Alice to optimize the mean number of photons as a function of the distance. The SARG04 protocol is found to perform better than BB84, both in secret-key rate and in maximal achievable distance, for a wide class of Eve's attacks.
Abstract The SARG04 quantum key distribution protocol can offer greater robustness against photon number splitting attacks than the BB84 protocol that is implemented with weak pulses. In this paper, we propose a tight key analysis for the SARG04 protocol, by considering the one-decoy method and investigating its performance under the influence of a detector afterpulse. Our results demonstrate that an increase in block size leads to a slight increase in both the secure key rate and the maximum transmission distance. Importantly, the detector afterpulse plays a crucial role in practical applications and has a more pronounced effect on the SARG04 protocol compared to the BB84 protocol.
Cryptography in the modern era is very important to prevent a cyber attack, as the world tends to be more and more digitalized. Classical cryptographic protocols mainly depend on the mathematical complicacy of encoding functions and the shared key, like RSA protocol in which security depends upon the fact that factoring a big number is a hard problem to the current computers. This means that high computing power can help you crack traditional encryption methods. Quantum machines claim to have this kind of power in many instances. Factorization of big numbers may be possible with Shor's algorithm with quantum machines in considerable time. Apart from this, the main problem is key sharing i.e., how to securely share the key the first time to validate the encryption. Here comes quantum key distribution. Two parties who are interested in communication with each other, create a process, which claims considerable security against an eavesdropper, by encoding and decoding information in quantum states to construct and share a secret key. Quantum key distribution may be done in a variety of ways. This paper begins with experimental verification of the BB84 procedure utilizing four bases (using phase gates) followed by the experimental realization of the SARG04 protocol which was derived from BB84 Protocol to overcome PNS attack. The possibility of a third-party attack and the effect of noise is considered and implemented. The IBM Quantum Experience platform was used for all of the implementations.
Quantum Key Distribution (QKD) is an enabling technology to current modern cryptography which utilizes nature properties of light to transfer key information safely over an unsecured channel. Via this application, both communicating parties can share an identical secret key for their chosen cryptographic scheme in various applications which demand a protection to their highly important message. A designated key bits reconciliation method is known as QKD protocol. The first ever protocol introduced and popularly used is the Bennett and Brassard 1984 (BB84) protocol. However the implementation with current technology has restricted its primary capability to guarantee the security of secret key establishment. As a result, it is vulnerable to Eavesdropping attack. This leads many works on practical protocol and the most significant contribution having used the same setup as the BB84 protocol is the Scarani, Acin, Ribordy and Gisin 2004 (SARG04) protocol. The SARG04 is more robust against photon number splitting attacks and double the BB84 critical transmission distance. Nevertheless SARG04 protocol still has lower percentage of sifted key bit compared to BB84 protocol. Therefore this study has embarked its objectives on identifying and proposing a new QKD protocol which can improve the robustness while allowing higher final key length. The alternative protocol is designed by combining the existing SARG04 decoding with improved SARG04 decoding (ISARG04). Unlike SARG04 which discards any inconclusive result from his measurement, ISARG04 will always accept the non orthogonal state with half probability of error. The robustness of the proposed protocol is simulated based on simple Intercept Resend attack and photon number splitting attacks in quantum key distribution. This new protocol is robust up to two photons per pulse and has an improvement to secure transmissions of bits at higher link loss, up to 26.7 dB. In summary, this work has presented an improved discrete variable protocol which improved the sifted key length and robust to eavesdropping. It is hope that, this contribution might be a breakthrough for near future QKD protocol.