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What Physical Layer Security Can Do for 6G Security
Miroslav Mitev, Arsenia Chorti, H. V. Poor, Gerhard Fettweis
TL;DR
The paper addresses unresolved B5G and 6G access-network security challenges in demanding use cases such as mMTC, URLLC, and autonomous cyberphysical systems. It reviews a bottom-up physical-layer security approach and its connections with cryptography across key generation, wiretap channels, authentication, privacy, and jamming. The review concludes that PLS offers lightweight, adaptive, and complementary security possibilities for future wireless systems.
Problem
Strengthened 5G protocols still leave B5G access-network security issues, while low-latency and massive-IoT use cases require security approaches with limited complexity and footprint.
Method
The paper provides a comprehensive review of PLS foundations, applications, and crypto-PLS connections, covering shared-randomness key generation, wiretap channels, authentication, privacy, and jamming.
Results
The review shows that wireless-channel properties can provide secrecy advantages, support lightweight key generation and authentication, and motivate PHY-layer defenses against emerging attacks.
Takeaways & Limitations
PLS can serve as a lightweight, adaptive security layer for future wireless systems, complementing conventional cryptography and supporting low-latency and massive-IoT use cases.
Abstract
from arXiv · showhide
While existing security protocols were designed with a focus on the core network, the enhancement of the security of the B5G access network becomes of critical importance. Despite the strengthening of 5G security protocols with respect to LTE, there are still open issues that have not been fully addressed. This work is articulated around the premise that rethinking the security design bottom up, starting at the physical layer, is not only viable in 6G but importantly, arises as an efficient way to overcome security hurdles in novel use cases, notably massive machine type communications (mMTC), ultra reliable low latency communications (URLLC) and autonomous cyberphysical systems. Unlike existing review papers that treat physical layer security orthogonally to cryptography, we will try to provide a few insights of underlying connections. Discussing many practical issues, we will present a comprehensive review of the state-of-the-art in i) secret key generation from shared randomness, ii) the wiretap channel and fundamental limits, iii) authentication of devices using physical unclonable functions (PUFs), localization and multi-factor authentication, and, iv) jamming attacks at the physical layer. We finally conclude with the proposers' aspirations for the 6G security landscape, in the hyper-connectivity and semantic communications era.
I. INTRODUCTION
The paper argues that 6G security must extend beyond strengthened 5G protocols by incorporating physical-layer mechanisms into access-network protection. It reviews how PLS can support confidentiality, authentication, lightweight key agreement, and resilience against emerging attacks in demanding use cases.
- Motivation: B5G scenarios such as URLLC, mMTC, and autonomous cyberphysical systems introduce security challenges that classical mechanisms may not fully address.These include false base-station attacks, URLLC security issues, and the need for additional protection layers.
- Key generation: Wireless channels can support key agreement because reciprocal, time-varying fading provides shared randomness during channel coherence time.This is particularly relevant to IoT applications requiring strong but lightweight key-agreement mechanisms.
- Authentication: PLS-based authentication options include PUFs, wireless fingerprinting, and high-precision localization, potentially combined with classical methods for D2D and Industry 4.0 settings.The 6G network-of-networks vision may require separate authentication for local subnetworks and the core network.
- Physical-layer threats: Emerging attacks such as jamming and pilot contamination during beam allocation or network entry require PHY-layer responses, including jamming-resilient waveform and code design.The paper also discusses wiretap-channel scenarios enabled by highly directional mmWave and subTHz beamforming.
- PLS rationale: PLS can provide information-theoretic security with lightweight mechanisms and can complement conventional approaches as an additional RAN security layer.The paper also presents hybrid crypto-PLS protocols as potentially fast, low-footprint, and low-complexity solutions.
- Review scope: The paper frames its review around security goals, system models, assumptions, and desirable properties before examining PLS and cryptographic connections.Its broader motivation is to develop adaptive, context-aware, and potentially low-cost security controls from the physical layer upward.
A. Confidentiality
This section introduces confidentiality mechanisms from classical cryptography, including one-time pads, stream ciphers, AES, authenticated encryption, and certificates. It emphasizes the trade-off between perfect secrecy, practical key requirements, semantic security, and integrity protection.
- Perfect secrecy: Perfect secrecy is achievable when the secret-key entropy is at least as large as the plaintext entropy, as in the one-time pad.The one-time pad XORs plaintext with the key but requires a key at least as long as the data.
- Stream ciphers: Stream ciphers make encryption practical by expanding short keys into pseudorandom sequences, but they do not provide perfect secrecy because processing cannot increase entropy.Their security is instead associated with semantic security and computational indistinguishability from random output.
- Block ciphers: AES is a semantically secure symmetric block cipher that maps an n-bit plaintext to an n-bit ciphertext using a key selected from 128, 192, or 256 bits.AES-256 is identified in the passage as quantum resistant.
- Integrity: Message authentication codes protect data integrity by appending a secret-key-based tag that the receiver verifies.The section contrasts modern MACs with historically broken constructions based on linear functions.
- Authenticated encryption: Authenticated encryption combines confidentiality and integrity, with encrypt-then-sign presented as a provably secure approach.The receiver verifies the ciphertext tag before proceeding to decryption.
- Authentication: Digital certificates use signatures from a trusted certificate authority to establish the authenticity of their owners.Certificates contain information about the authority, owner, and validity period.
III. MOTIVATION FOR CONSIDERING PHYSICAL LAYER SECURITY
Physical layer security is motivated by the latency, scale, and complexity constraints of emerging networks, where conventional cryptographic mechanisms may be too heavy or incomplete. The section reviews wiretap coding and finite-blocklength results as ways to provide confidentiality through channel properties and joint reliability-security coding.
- Motivation for considering PLS: PLS is considered because conventional security mechanisms face stringent latency, low-footprint, and complexity requirements in emerging technologies.The motivation includes faster authentication and integrity checks, lightweight protection for massive numbers of devices, and concerns about increasingly heavy post-quantum mechanisms.
- Confidential transmission: Wiretap coding provides confidential transmission by exploiting differences between Bob’s and Eve’s noisy channels, potentially without secret keys.Positive secrecy capacity requires Eve’s channel to be degraded relative to Bob’s, such as when Bob has higher SNR.
- Confidential transmission: Physical propagation can create secrecy advantages through fading, artificial interference or jamming, and spatial diversity from antennas or relays.These mechanisms exploit natural or engineered degradation and additional secrecy degrees of freedom.
- Confidential transmission: In Gaussian broadcast channels, secrecy rates increase as Eve’s SNR decreases, but secrecy collapses when Eve’s SNR reaches Bob’s 10 dB level.The broadcast-channel analysis compares common and secrecy rates as Eve’s channel quality changes.
- Confidential transmission: Rayleigh fading can preserve secrecy capacity even when Bob and Eve observe statistically identical channels, because fading provides time-varying degrees of freedom.Under perfect channel knowledge and σ2 = 1, secrecy does not collapse as it does in the non-fading Gaussian case.
- Confidential transmission: Finite-blocklength analysis is necessary for low-latency security because asymptotic secrecy results are unsuitable for short packets and can leave substantial achievability gaps.For the Gaussian wiretap channel with Bob at 3 dB and Eve at −3 dB, the finite-blocklength upper bound remains far from asymptotic secrecy capacity.
B. Privacy in sensing systems
Privacy in sensing systems requires balancing the usefulness of shared data against information leakage. The paper describes trade-offs modeled through distortion, reverse water-filling, control, and distributed source coding.
- Privacy and usefulness are fundamentally opposed: fully private data is highly distorted, while fully accessible data has zero equivocation and no privacy.
- In smart-meter sensing, transmitted electricity-use data provides utility but can reveal users’ in-home activities to the power supplier.
- A reverse water-filling model suppresses signals below threshold φ while transmitting stronger signals, letting φ determine the privacy sacrificed for utility.
- A control-based model uses storage and energy harvesting to characterize the utility-privacy frontier through wasted power versus information leakage.
- Competitive privacy models multiple agents that share measurements to improve estimation while limiting their own information leakage.
- Information theory connects privacy analysis to practical directions including finite-blocklength communication and large-network scaling laws.
V. SECRET KEY GENERATION USING PLS
The section presents secret key generation as a three-part process and discusses how it can be integrated into cryptographic systems and protected against active attacks.
- Secret key generation is examined through shared-randomness extraction, integration with cryptographic systems, and resilience to active attacks.
A. Secret key generation
Secret key generation uses reciprocal channel randomness to produce shared keys through advantage distillation, information reconciliation, and privacy amplification. The section also addresses measurement components, active-attacker resilience, and reconciliation-code performance.
- Alice and Bob exchange pilots, quantize correlated observations, reconcile the resulting bits, and apply privacy amplification to obtain an adversary-unobservable key.
- Reconciliation-code performance is compared against an information-theoretic upper bound for n = 128.
- Large-scale fading is location- and distance-dependent, so key extraction should remove it or compress it more heavily to retain unpredictable small-scale components.
- Polar codes with CRC and BCH codes with list decoding outperform other tested reconciliation approaches for n = 128 and n = 512.
B. Secret key generation in hybrid crypto systems
The paper combines secret-key generation with encryption and authentication to form hybrid physical-layer security schemes. It then frames transmission of encrypted data and reconciliation information as a constrained optimization problem, where parallel transmission can improve efficiency.
- Hybrid authenticated encryption: A hybrid scheme combines secret-key generation, symmetric encryption, and message authentication into one authenticated-encryption protocol.The generated key is split into encryption and integrity portions, while side information, ciphertext, and authentication tags are transmitted together.
- Transmission optimization: The transmission design partitions subcarriers into data and side-information subsets and imposes cryptographic, reconciliation-rate, power, and channel-capacity constraints.The cryptographic constraint relates key-generation and data rates, while the information-theoretic constraint relates syndrome and key-generation rates.
- Transmission optimization: The resulting optimization problem can be solved optimally by dynamic programming or sub-optimally with heuristic methods.The formulation illustrates how physical-layer parameters can be connected to cryptographic schemes for cross-layer optimization.
- Efficiency results: The parallel approach generally outperforms sequential transmission, while its efficiency decreases as β increases because more frequent key generation leaves less capacity for data.The parallel heuristic achieves efficiency equivalent to the optimal dynamic-programming solution.
- Efficiency results: Under imperfect channel estimates, parallel transmission also outperforms sequential transmission across delay-related θ and key-generation-related β values.As θ approaches zero, the optimal power allocation converges to water-filling; as θ approaches infinity, it converges to total channel inversion.
C. Secret key generation under active attacks
The section examines active attacks on secret-key generation, especially pilot-signal injection by a man-in-the-middle attacker. Randomized probing can convert injection into uncorrelated jamming, preserving secrecy at the cost of key rate, while reactive-jamming outcomes depend on sensing thresholds and legitimate-user power.
- Injection attacks: A man-in-the-middle attacker can use precoding to inject an equivalent signal into Alice’s and Bob’s observations when equipped with an extra antenna.The injected signal becomes part of the shared randomness used for key generation.
- Injection attacks: Injection changes the shared randomness from XH to XH + W, allowing Mallory to obtain partial information about the generated key.The attack is therefore more damaging than ordinary uncorrelated interference.
- Countermeasures: Independent randomized probes X and Y allow Alice and Bob to remove the injected component from their shared randomness when X and Y are uncorrelated.The resulting attack is reduced to uncorrelated jamming, which affects achievable key rate without revealing the key to Mallory.
- Reactive jamming: A reactive jammer senses transmissions and selectively jams subcarriers above a chosen detection threshold, creating a hierarchical leader-follower game.The Stackelberg equilibrium depends on the jammer’s receiver sensitivity and the legitimate users’ available power.
- Reactive jamming: If the jammer detects all ongoing transmissions, Alice and Bob must transmit at full power.With a poorly chosen threshold, legitimate users may instead distribute power below the sensing threshold or transmit at full power across all subcarriers.
VI. AUTHENTICATION USING PLS
Physical-layer security is presented as a way to address authentication challenges created by increasing cryptographic complexity and stringent latency requirements. The section surveys PUFs, localization, and a multi-technology 0-RTT authentication protocol, while noting replay vulnerability in the cryptographic 0-RTT example.
- Authentication motivation: Authentication schemes face a trade-off between cryptographic key size and delay, motivating physical-layer alternatives.The paper frames PLS authentication as a response to increasing complexity and latency demands.
- Authentication trade-offs: TLS 1.3 0-RTT session resumption reduces repeated authentication delay by using resumption keys, but the scheme is vulnerable to attacks such as replay.The paper presents this vulnerability as a limitation accompanying the latency benefit.
- Authentication approaches: The section reviews PUFs, localization-based authentication, and a secure 0-RTT protocol combining multiple physical-layer technologies.These mechanisms are presented as possible authentication approaches for 6G systems.
A. Physical unclonable functions
PUFs use manufacturing-induced device variations as device fingerprints for authentication. A standard PUF protocol separates offline enrollment, which records challenge-response information and helper data, from online authentication.
- PUF principle: PUFs exploit device-specific variations in delays, power-on state, and jitter to support authentication.These variations arise from manufacturing conditions such as temperature and vibrations, while devices otherwise operate similarly.
- PUF protocol: PUF authentication has offline enrollment and online authentication phases.During enrollment, challenges are applied to the PUF, noisy measurements are encoded into helper data, and a verifier stores challenge-response pairs.
B. Location-based authentication
Location-based authentication can use lightweight RSS-based proximity estimation to authenticate access points with low-end devices, while remaining vulnerable to interference and localization uncertainty.
- A low-complexity proximity estimator uses RSS measurements from multiple positions to let a mobile device authenticate an access point.The device moves unpredictably, uses a premise map and known access-point locations, and requires only a single antenna with low computational power.
- The approach avoids multiple simultaneous RSS-measuring nodes and advanced hardware or computation while still serving as an authentication factor.
- Proximity detection can provide resilience to impersonation attacks involving malicious access points.
- Deployment requires addressing jamming, man-in-the-middle attacks, interference, pilot contamination, and uncertainty in localization measurements.Measurement variability can change with operating distance, affecting the trustworthiness and precision of localization information.
C. Multi-factor PLS authentication
Multi-factor physical-layer authentication combines credentials such as PUFs, shared-key generation, and location fingerprints within a broader adaptive 6G security vision. The approach is promising for lightweight security, but depends on characterizing channel predictability and deploying context-sensitive security levels.
- Multi-factor PLS authentication: A multi-factor PLS authentication protocol combines PUFs, secret-key generation, and location fingerprints for device authentication.PUFs provide a mutual authentication factor between a mobile node and a static server through enrolment and authentication phases.
- Multi-factor PLS authentication: Physical-layer authentication research includes high-entropy PUFs and accurate, privacy-preserving location-based authentication as open design problems.
- Adaptive 6G security: PLS is presented as an adaptive 6G security layer whose secrecy outage probability can be tuned by adjusting transmission rate.
- Adaptive 6G security: Wireless channels can provide uniqueness for authentication or entropy for key generation, depending on their temporal variability.Slow fading can support uniqueness, whereas rapidly changing small-scale fading can provide entropy.
- Open research issues: An open research question is determining which channel properties are predictable or unpredictable across time, frequency, and space.This characterization is intended to align PLS metrics with semantic security metrics.
- Security-level design: The paper proposes defining security levels from information criticality, attacker value, system resilience, threat level, and QoS constraints.
- Security-level design: PLS offers concrete use cases beyond encryption and lightweight security solutions for low-latency and massive IoT systems.The paper emphasizes characterizing and exploiting the wireless channel from a security perspective.