In this study, we aim to further advance solutions to critical challenges in physical layer secure key generation and distribution (PLSKGD) for optical fiber communication, particularly the limitations of traditional information reconciliation (IR) protocols in mitigating initial key inconsistencies caused by noise and interference. We introduce two innovative approaches: a check node correlation coefficient-based early termination (CNCC-ET) scheme and an elastic reconfigurable (ER) scheme to enhance error correction performance and accelerate IR. The CNCC-ET scheme dynamically monitors the correlation coefficient of messages between check and variable nodes, enabling early termination of iterations when decoding is likely to fail, thus significantly reducing decoding complexity. In high bit error rate (BER) regions, this scheme achieves up to a 51.8% reduction in average iteration counts while maintaining low frame error rates (FER). Concurrently, our ER scheme adapts to varying BER conditions by dynamically adjusting the ratio of information to check bits, achieving a remarkable 90.26% FER reduction in high-BER scenarios. Hardware implementation on a Xilinx Virtex-7 FPGA validates the feasibility of these methods, demonstrating a 47.2% reduction in IR time overhead and significantly improved error correction capability. This study advances the development of more secure and efficient optical fiber communication systems, with potential applications extending to other physical layer security contexts.
In this study, we aim to further advance solutions to critical challenges in physical layer secure key generation and distribution (PLSKGD) for optical fiber communication, particularly the limitations of traditional information reconciliation (IR) protocols in mitigating initial key inconsistencies caused by noise and interference. We introduce two innovative approaches: a check node correlation coefficient-based early termination (CNCC-ET) scheme and an elastic reconfigurable (ER) scheme to enhance error correction performance and accelerate IR. The CNCC-ET scheme dynamically monitors the correlation coefficient of messages between check and variable nodes, enabling early termination of iterations when decoding is likely to fail, thus significantly reducing decoding complexity. In high bit error rate (BER) regions, this scheme achieves up to a 51.8% reduction in average iteration counts while maintaining low frame error rates (FER). Concurrently, our ER scheme adapts to varying BER conditions by dynamically adjusting the ratio of information to check bits, achieving a remarkable 90.26% FER reduction in high-BER scenarios. Hardware implementation on a Xilinx Virtex-7 FPGA validates the feasibility of these methods, demonstrating a 47.2% reduction in IR time overhead and significantly improved error correction capability. This study advances the development of more secure and efficient optical fiber communication systems, with potential applications extending to other physical layer security contexts.
The information reconciliation (IR) stage is a critical component of post-processing in continuous-variable quantum key distribution (CV-QKD), which is essential to ensure high secret key rates and long transmission distances. Currently, improving reconciliation efficiency is a key research focus in the CV-QKD field. Additionally, the low-density parity-check (LDPC) decoding speed in IR is a post-processing bottleneck in high-speed CV-QKD. In this paper, we propose a construction scheme for spatially coupled (SC) LDPC codes in CV-QKD systems and maintain a constant code rate when extending the code length as required. Furthermore, we investigate the relationship between the syndrome variation pattern (SVP) and the frame error rate. Adaptively adjusting the number of decoding iterations according to the distinct convergence behaviors of the SVP, the convergence-based early termination (C-ET) scheme is proposed, which can reduce invalid iterations. The simulation results show that the proposed construction scheme for SC-LDPC codes can improve the reconciliation efficiency to 97% and achieve a flexible code length. The C-ET scheme achieves a 292% improvement in information throughput compared to the original ET scheme when the reconciliation efficiency reaches 97%. Moreover, the C-ET scheme also achieves a reduction in decoding latency compared to the existing syndrome-based early termination scheme. These advances provide a substantial step toward improving reconciliation efficiency and breaking the post-processing bottleneck in CV-QKD systems.
Information reconciliation (IR) is an indispensable component in the post-processing stage of continuous-variable quantum key distribution (CV-QKD), which adopts error-correcting codes to address the asymmetry of secret keys. Currently, low-density parity-check (LDPC) decoding in IR is a post-processing bottleneck in high-speed CV-QKD systems since the upper bound on secret key rate is higher than the information throughput delivered by decoder. In this paper, we study the relationship between the syndrome variation pattern (SVP) in iterative decoding and reconciliation frame error rate. An early termination scheme based on SVP is proposed and applied to multidimensional reconciliation, which can increase information throughput by adaptively adjusting the iteration number of iterative decoding to real-time decoding status. Furthermore, we show that only the resulting syndrome of the highest-rate code part in Raptor-like LDPC codes needs to be calculated to verify whether the reconciliation is successful by studying the convergency of resulting syndrome, which can save a large fraction of computational resources for syndrome calculation. Simulation results show that information throughput of the proposed scheme can be improved by 617.1% compared to the existing scheme when the IR efficiency reaches 97.09%. The proposed scheme points out a new direction for breaking the post-processing bottleneck in high-speed CV-QKD systems.
Physical-layer key generation technology requires information reconciliation to correct channel estimation errors between two legitimate users. However, sending the reconciliation signals over the public channel increases the communication overhead and the risk of information leakage. Aiming at the problem, integrated secure communication schemes using non-reconciled keys have attracted extensive attention. These schemes exploit channel coding to correct both inconsistent keys and transmission error bits. Meanwhile, more redundant code bits must be added to correct errors, which results in a lower secure transmission rate. To address the problem, we analyze the merit of channel correlation between non-reconciled key generation and secure transmission. Inspired by this, we propose a non-reconciled physical-layer keys-assisted secure communication scheme based on channel correlation. First of all, the signal frame is designed to make use of channel correlation between non-reconciled key generation and secure transmission. Based on the channel correlation, non-reconciled keys are then generated from the wireless channel to encrypt transmitted data. Moreover, an adaptive coding algorithm based on the equivalent channel is presented to encode the data bits before encryption, to guarantee reliable transmission. Finally, theoretical analysis and simulations demonstrate the significant performance of the proposed scheme in terms of low bit error ratio and high secure transmission rate.
This paper proposes an end-to-end physical layer secure key generation and distribution (E2E-PLSKGD) scheme based on error vector phase (EVP) for fiber transmission systems. It addresses compatibility issues in existing PLSKGD caused by additional random sources or devices, as well as limitations in application scenarios and insufficient key generation rate (KGR) from amplitude or phase modulation of intensity-modulated signals. We introduce a position-enhanced transformer (PE-transformer) architecture with adaptive position coding (APC) for accurate E2E modeling in coherent optical systems. Leveraging the inherent high randomness of signal phase in fiber systems, legitimate parties use the PE-transformer to build E2E models via bidirectional transmission, compute error vectors, and extract EVP noise as key quantization sequences. Using data from new negotiation cycles and transfer learning (TL), local E2E source models are rapidly updated to target models, enhancing key security. Simulations show superior E2E modeling, with R-Square values >0.95, excellent waveform fitting, and consistent data distributions across distances. Pearson correlation coefficients (CC) of EVP sequences between legitimate parties exceed 0.8 across 50-80 km distances and -2 dBm to 4 dBm launch powers, indicating strong correlation. Experimental validation on a 120 km fiber system shows an EVP sequence correlation of 0.72, with real-time post-processing achieved via FPGA. At a 5G baud rate, error-free KGR reaches 0.8 Gbps. The proposed architecture enables precise E2E modeling, offering a new approach for dynamic, high-quality key generation and a potential method for AI-empowered fiber transmission security.
This letter proposes an efficient information reconciliation coding scheme for physical layer secret key generation. We consider a high-rate key generation system, where multiple key bits are extracted from one Gaussian random variable source, each of which has different reliability depending on its bit level. Although the conventional bit-interleaved coded modulation (BICM)-type reconciliation protects all the key bits by forward error correction (FEC), the computational complexity associated with its encoding and decoding can be a bottleneck in achieving a high secret key rate especially for low-cost devices with limited computational capabilities. To alleviate this complexity issue, we propose a new reconciliation scheme that efficiently reduces the complexity by applying FEC only to the least reliable bits (LRBs). We demonstrate by simulations that the proposed information reconciliation based on partial encoding achieves a lower key bit mismatch rate (BMR) than the conventional BICM-type reconciliation even with lower decoding complexity. Furthermore, we provide a lower bound on the asymptotic BMR of the proposed scheme that is useful for estimating the performance in high signal-to-noise ratio (SNR) regimes.
The focus of this research activity is to work on pragmatic information reconciliation applied to QKD schemes based on single photon or weak pulse laser (WPL) sources, so as to use feed-forward techniques which minimize the interaction between transmitter and receiver. The core ideas of the thesis are employing Forward Error Correction (FEC) coding as opposed to two-way communication for information reconciliation in QKD schemes, exploiting all the available information for data processing at the receiver including information available from the quantum channel, since optimized use of this information can lead to significant performance improvement, and providing a security versus secret-key rate trade-off to the end-user within the context of QKD systems. Moreover, as shown by accurate experimental studies, the communication channel used for quantum key exchange is not able to reach high levels of reliability (the Quantum Bit Error Rate -QBER may have a high value), both because of the inherent characteristics of the system, and of the presence of a possible attacker. In order to obtain acceptable residual error rates, it is necessary to use a parallel classical and public channel, characterized by high transmission rates and low error rates, on which to transmit only the redundancy bits of systematic channel codes with performance possibly close to the capacity limit. Furthermore, since the more redundancy is added by the channel code, the more the corresponding information can be used to decipher the private message itself, it becomes necessary to design high-rate codes obtained by puncturing a low-rate mother code, possibly achieving a redundancy such that elements of the secret message cannot be uniquely determined from the redundancy itself, so for that purpose we designed high rate LDPC codes. Using high rate codes increases the security with trade-off to performance. Other low photon number applications have also been considered, such as weak-laser pulses (WLP) communication. For that purpose, a low-complexity photon-counting receiver has been considered which may be employed in long-distance amplification-free classical optical communication schemes, and which is typically modeled as an equivalent Binary Symmetric Channel (BSC). We have developed a time varying Binary Input-Multiple Output (BIMO) channel model for this low-complexity photon-counting receiver, and analyzed its performance in presence of soft-metric based capacity approaching iteratively decoded error correcting codes, such as soft-metric based Low Density Parity Check (LDPC) codes and polar codes. We show that the classical channel capacity of the suggested BIMO model is higher than the capacity of the BSC model, and that the use of the BIMO model allows to feed the channel decoder with soft information, in the form of Log-Likelihood Ratios (LLRs), achieving a significant reduction in Bit Error Rate (BER) and Frame Error Rate (FER) with respect to classical hard-metric-based schemes which should be used in conjunction with a BSC channel model.