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International Conference on Communications

Secret Key Generation in Multi-Mode Fiber Channels: Channel Measurements and Achievable Rates

Pin-Hsun Lin, Paul Nowitzki, Eduard Axel Jorswieck, Dennis Pohle, Jürgen W. Czarske

Published Jun 9, 2024• 0 citations
Publisher

Abstract

Secret keys play a critical role in secure data transmission, and an efficient secret key generation scheme will enhance the secure transmission rate. However, the secret key is a scarce resource for classical encryption schemes like the Rivest-Shamir-Adleman (RSA) cryptosystem and its counterpart in quantum applications, and we should extract it from all possible random sources to maximize the key rate. In this paper, we investigate the achievable rate of the secret key generation (SKG)-channel-model based on an actual measured multi-mode fiber (MMF) channel, where there is an additional public discussion channel between the legitimate parties, which the adversary can perfectly overhear. In particular, we measure the fiber transmission matrix by digital holography. Based on the real measurements, we show that for 55-mode MMF, we can achieve a positive secret key rate at 51.3 bit/channel use with mode-dependent loss (MDL) at 1 dB and transmit power at 20 dBm. This demonstrates the feasibility of SKG in MMF.

DOI
10.1109/icc51166.2024.10622562

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Secret keys play a critical role in secure data transmission, and an efficient secret key generation scheme will enhance the secure transmission rate. However, the secret key is a scarce resource for classical encryption schemes like the Rivest-Shamir-Adleman (RSA) cryptosystem and its counterpart in quantum applications, and we should extract it from all possible random sources to maximize the key rate. In this paper, we investigate the achievable rate of the secret key generation (SKG)-channel-model based on an actual measured multi-mode fiber (MMF) channel, where there is an additional public discussion channel between the legitimate parties, which the adversary can perfectly overhear. In particular, we measure the fiber transmission matrix by digital holography. Based on the real measurements, we show that for 55-mode MMF, we can achieve a positive secret key rate at 51.3 bit/channel use with mode-dependent loss (MDL) at 1 dB and transmit power at 20 dBm. This demonstrates the feasibility of SKG in MMF.
Secret keys are critical for many security mechanisms including secure data transmission. An efficient secret key generation and sharing scheme enhances the secure transmission rate. Meanwhile, secret keys are scarce resources for classical encryption schemes like the Rivest-Shamir-Adleman (RSA) cryptosystem and its post-quantum security counterparts in quantum applications. Therefore, harnessing every available random source for secret key extraction is crucial for maximizing the key rate across a range of potential applications. In this paper, we investigate the achievable rate of the secret key generation (SKG)-channel-model based on an actual measured multi-mode fiber (MMF) channel, where there is an additional public discussion channel between the legitimate parties, which the adversary can perfectly overhear. In particular, we first measure the MMF transmission matrix using digital holography, then design power allocation schemes among the modes. Our schemes can achieve a positive secret key rate at 51.3 bit per channel use, under a mode-dependent loss of 1 dB and a transmit power of 20 dBm. In addition, we implement an SKG system via MMF by explicitly designing the wiretap codes and discussion protocol. More specifically, we design the hash function and point-to-point channel encoder/decoder for each mode in the MMF based on Hayashi’s wiretap coding. The designed system can achieve a secret key rate-leakage trade-off atRSK=23.6R_{SK} = 23.6RSK​=23.6bits per channel use and an average leakage rate at$ 4.22 \cdot 10^{-3}bitsperchannelusepermode,atthetransmitpowerbits per channel use per mode, at the transmit powerbitsperchannelusepermode,atthetransmitpower\textsf {P}_{T}$at 50 dBm and mode-dependent loss at 1 dB with the error probability constraint at Bob as10−510^{-5}10−5. This demonstrates the feasibility and effectiveness of generating SKG via MMF, aligning with the common approaches of post-quantum cryptography and quantum key distribution in addressing the challenges posed by emerging quantum computers.
Reliable and secure communication is an important aspect of modern fiber optic communication. In this work we consider a multi-mode fiber (MMF) channel wiretapped by an eavesdropper. We assume the transmitter knows the legitimate channel, but statistical knowledge of the eavesdropper's channel only. We propose a transmission scheme with artificial noise (AN) for such a channel. In particular, we formulate the corresponding optimization problem which aims to maximize the average secrecy rate and develop an algorithm to solve it. We apply this algorithm to actual measured MMF channels. As real fiber measurements show, for a 55 mode MMF we can achieve positive average secrecy rates with the proper use of AN. Furthermore, the gain compared to standard precoding and power allocation schemes is illustrated.
The emergence of Internet of things (IoT) applications brings the challenge of finding lightweight schemes for secret key distribution. A promising solution coming from the physical layer is the so called secret key generation (SKG) from shared randomness. SKG allows two communicating parties to extract the shared randomness already present in wireless channels. This work investigates the achievable SKG rates using received signal strength (RSS) as a key generation parameter. A multi-path scenario is considered and the probability density function of the RSS in different channel conditions is evaluated. Next, through a numerical evaluation, the SKG rates are derived in the form of mutual information (MI) estimates, using a 3GPP standard channel model. The simulations are performed for different values of bandwidth and delay spread. We demonstrate that while both parameters have similar impact on how multi-path components are resolved, their effect on the MI is opposite.
The sixth generation of wireless networks (6G) is expected to support the deployment of Internet of things (IoT) devices in massive scales. Finding lightweight and decentralized secret-key distribution primitives is therefore a challenge. Secret-key generation (SKG) from wireless channel coefficients is seen as a possible solution. It allows the extraction of secret keys using the channel randomness observed at the physical layer, without a centralized key distribution server. In this work an SKG approach suitable for wideband IoT devices is proposed. We investigate a filterbank-based SKG method, in which secret bits are generated through power measurements over different frequencies. To minimize dependencies and correlation among frequencies the quantile and the Karhunen-Loeve transforms are used. Finally, we perform a numerical evaluation of the achievable SKG rates, in the form of mutual-information (MI) estimates, using 3GPP channel models. Our numerical evaluation shows that the achievable SKG rate depends on the channel statistics, and, hence, to optimally harvest the information, devices need to be channel-aware.
With the growing demand for service access and data transmission, security issues in optical fiber systems have become increasingly important and the subject of increased research. Physical layer secret key generation (PL-SKG), which leverages the random but common channel properties at legitimate parties, has been shown to be a secure, low-cost, and easily deployed technique as opposed to computational-based cryptography, quantum, and chaos key methods that rely on precise equipment. However, the eavesdropper (Eve) potential for current PL-SKG in fiber communications has been overlooked by most studies to date. Unlike wireless communications, where the randomness comes from the spatial multi-paths that cannot be all captured by Eves, in fiber communications, all the randomness (from transmitted random pilots or channel randomness) is contained in the signals transmitted inside the fiber. This, therefore, enables a tapping Eve to reconstruct the common features of legitimate users from its received signals, and further decrypt the featured-based secret keys. To implement this idea, we designed two Eve schemes against polarization mode distortion (PMD) based PL-SKG and the two-way cross multiplication based PL-SKG. The simulation results show that our proposed Eves can successfully reconstruct the legitimate common feature and the secret key relied upon, leading to secret key rate (SKR) reductions of between three and four orders of magnitude in the PL-SKG schemes studied. As a result, we reveal and demonstrate a novel eavesdropping potential to provide challenges for current physical layer secret key designs. We hope to provide more insightful vision and critical evaluation on the design of new physical layer secret key schemes in optical fiber links, to provide more comprehensively secure, and intelligent optical networks.
The Secret Key Generation (SKG) scheme that exploits the reciprocity and uniqueness of wireless channel between two users plays a significant part in a new increasing distributed security system. The scheme performance can be distinguished based on the low value of Key disagreement Rate (KDR), the high value of Key Generation Rate (KGR), as well as the fulfillment of the NIST randomness standard. The previous SKG scheme has a high KDR due to a direct quantization of a measurement result of the Received Signal Strength (RSS). To overcome the above issue, we conduct a pre-processing of measurement result before quantization with the Kalman method. The pre-process is carried out to improve the channel reciprocity between two legitimate users with the objective to reduce the bit mismatch. Through an experiment, we propose a new quantization scheme called a Modified Multi-Bit (MMB) that uses a multi-bit system on every level of quantization. The test results show that the proposed combination of preprocessing and the MMB scheme has a better performance compared to the existing schemes in terms of KDR and KGR. The Secret Key generated by our scheme also fulfills the NIST randomness standard.