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IEEE Photonics Technology Letters

Efficient Post-Processing for Physical-Layer Secure Key Distribution in Fiber

Shuhang Chen, Liuming Zhang, Weisheng Hu, Xuelin Yang

Published Feb 22, 2021• 12 citations
Publisher

Abstract

Post-processing is an essential step towards the practical deployment of physical-layer secure key distribution (SKD) in classical fiber channels. However, the comparison and combination of different post-processing methods are not explored yet. Here, we fully evaluate the post-processing protocol and analyze various combinations of quantization and information reconciliation (IR), and present the efficient solutions associated with the key entropy source between the legal parties. In addition, the SHA3-256 hash function-based privacy amplification is exploited to further enhance the robust key secrecy and randomness. The results show that, the proposed protocol significantly increases the achievable key generation rate (KGR) up to 130-205% using the combination of multi-bit adaptive quantizer and IR with syndrome-decoding LDPC, compared with the previous post-processing using the same original key waveforms extracted from physical-layer SKD.

DOI
10.1109/lpt.2021.3061129

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Post-processing is an essential step towards the practical deployment of physical-layer secure key distribution (SKD) in classical fiber channels. However, the comparison and combination of different post-processing methods are not explored yet. Here, we fully evaluate the post-processing protocol and analyze various combinations of quantization and information reconciliation (IR), and present the efficient solutions associated with the key entropy source between the legal parties. In addition, the SHA3-256 hash function-based privacy amplification is exploited to further enhance the robust key secrecy and randomness. The results show that, the proposed protocol significantly increases the achievable key generation rate (KGR) up to 130-205% using the combination of multi-bit adaptive quantizer and IR with syndrome-decoding LDPC, compared with the previous post-processing using the same original key waveforms extracted from physical-layer SKD.
The unique and random physical-layer properties of optical fiber channels have been exploited for a simple and cost-effective secure key generation and distribution (SKGD). However, state-of-the-art SKGD approaches focused mainly on initial key extraction, while the post-processing protocol has to be explored to guarantee error-free operation, effective key generation rate (KGR) and high key secrecy. Here we propose a full post-processing protocol for SKGD in fiber networks. A lossy quantizer is optimized to achieve the trade-off between the KGR and key disagreement rate. The distributed source coding based information reconciliation is used to remove quantization errors. The privacy amplification is applied to improve the randomness and secrecy of the generated key using hashing functions. The post-processing protocol provides a SKGD solution of error-free, high randomness and high secrecy in fiber networks.
This paper presents the design and development of practical post-processing software for a general quantum key distribution system. The proposed algorithms consist of three main contributions: 1) channel parameter estimation for quantum bit error rates by a random key sampling method, 2) information reconciliation using rate-adaptive LDPC codes, that collaborate with identical key confirmation by employing a polynomial-based hash function, and 3) privacy amplification performed by a universal hash function, where the binary information is randomly chosen to construct the Toeplitz matrix corresponding to the condition of a secure secret key rate. The proposed post-processing algorithms were implemented and tested in an entangled quantum key distribution experiment based on the BBM92 protocol over a 11-meter free-space quantum channel link. The experiment was conducted over 100 cycles and was able to generate the average sifted key rate of 1,109.08 bits per second. The efficiency of the proposed algorithms enabled two legitimate parties to generate the identical secret key according to the information-theoretic security principles, where the average estimated quantum bit error rate was 2.0127%, the average efficiency of information reconciliation was 1.1919, and the average final secret key rate was 428.42 bits per second. The proposed algorithms could be used in the industrial quantum key distribution systems based on a discrete-variable protocol by optimizing the input parameters of the post-processing procedure, especially for high-speed applications.
Dynamic properties of optical signals in fiber channel provide a unique, random and reciprocal source for physical-layer secure key generation and distribution (SKGD). In this paper, an inherent physical-layer SKGD scheme is proposed and demonstrated, where the random source is originated from the dynamic fluctuation of the instant state of polarization (SOP) of optical signals in fiber. Due to the channel reciprocity, highly-correlated fluctuation of Stokes parameter of SOP is shared between the legal partners, where an error-free key generation rate (KGR) of 196-bit/s is successfully demonstrated over 25-km standard single-mode fiber (SSMF). In addition, an active polarization scrambler is deployed in fiber to increase the KGR, where an error-free KGR of 200-kbit/s is achieved.
Quantum key distribution (QKD), based on quantum mechanics, enables unconditionally secure key exchange even against adversaries with unlimited computing power. As the core of QKD post-processing, information reconciliation (IR) and privacy amplification (PA) ensure key consistency and security by correcting errors and removing leaked information. With the trend toward miniaturized and chip-based QKD systems, post-processing modules require highly integrated designs. FPGAs offer an ideal solution due to their low cost, power efficiency, and flexibility. This paper presents an FPGA-based QKD post-processing architecture using a polar codes-based appending IR algorithm, achieving reconciliation efficiency better than 1.1 under Eµ≥ 0.06 and Rm= 4. A convolution-structured Toeplitz hash module is developed for PA, supporting 53.13 Mbps data rate and 26.45 Mbps key rate with RPA= 0.5. The complete system is implemented on a Xilinx XCKU040 FPGA. Experimental results show support for 176.58 Mbps input, 36.59 Mbps real-time processing, and 17.56 Mbps key output, demonstrating strong performance and engineering suitability. The design features high integration and portability, offering a practical hardware solution for efficient QKD post-processing.
Post-processing is a significant step in quantum key distribution (QKD), which is used for correcting the quantum-channel noise errors and distilling identical corrected keys between two distant legitimate parties. Efficient error reconciliation protocol, which can lead to an increase in the secure key generation rate, is one of the main performance indicators of QKD setups. In this paper, we propose a multi-low-density parity-check codes based reconciliation scheme, which can provide remarkable perspectives for highly efficient information reconciliation. By testing our approach through data simulation, we show that the proposed scheme combining multi-syndrome-based error rate estimation allows for a more accurate estimation of the error rate as compared with random sampling and single-syndrome estimation techniques before the error correction, as well as a significant increase in the efficiency of the procedure without compromising security and sacrificing reconciliation efficiency.