Frozen-Tag-Based Physical-Layer Authentication Against User Interference

Abstract: Tag-based physical layer authentication (PLA) has garnered significant attention due to its low complexity and enhanced security. However, existing PLA schemes encounter two challenges. First, unintended user interference, which overlaps with the authentication signal, corrupts the tag and degrades authentication performance. Second, the vulnerability introduced by direct embedding of the raw tag exposes the tag to the adversary and degrades the security. To address these challenges, this paper proposes a novel frozen-tag-based PLA framework. Different from typical schemes that directly embed the uncoded tag into the signal, a well-designed frozen tag is inserted for authentication, where the frozen tag is generated based on the concept of polar codes with the anchor information as information bits and raw tags as frozen bits. Accordingly, the proposed PLA framework offers two principal advantages. First, the authentication performance is improved since the legitimate receiver can decode the frozen tag and mitigate unintended user interference. Second, the authentication process becomes indecipherable to the illegitimate receiver due to the concealment of the raw tags. Furthermore, we conduct a comprehensive analysis of the proposed framework in terms of robustness, security, and compatibility. Theoretical analysis and simulation demonstrate that the proposed frozen-tag-based PLA framework not only enhances the detection performance but also significantly degrades Eve's capability to estimate the raw tags.

• The paper presents a frozen-tag PLA framework that encodes raw authentication tags with polar codes to significantly enhance resilience against multiuser interference.
• It employs sparse embedding and hash-driven index selection to obfuscate tag positions, thereby mitigating eavesdropping and spoofing risks.
• Experimental results demonstrate improved detection probability and maintained message integrity, confirming its practical applicability in high-density IoT and vehicular networks.

Frozen-Tag-Based Physical-Layer Authentication Against User Interference: An Expert Perspective

Motivation and Context

The proliferation of 5G/6G-era wireless ecosystems—particularly in high-density IoT, IIoT, and vehicular networks—has accentuated vulnerabilities in conventional upper-layer authentication (ULA) protocols, which are encumbered by high computational complexity and latency. Physical-layer authentication (PLA), leveraging intrinsic, measurable signal properties, offers low overhead and information-theoretic security. However, typical tag-based PLA schemes are fundamentally limited in practical multiuser environments: (1) authentication tag robustness is severely degraded by user interference, and (2) directly embedding raw tags renders them susceptible to eavesdropping attacks. This paper presents a paradigm shift, proposing a frozen-tag PLA framework that exploits polar codes to encode tags, thereby enhancing resilience against interference and improving secrecy.

Problem Formulation

The central authentication scenario models $K$ simultaneous transmitters, with a legitimate signal from Alice superimposed with residual interference from other users at Bob’s receiver. Standard multiuser detection techniques, e.g., imperfect successive interference cancellation and quasi-orthogonal pilot schemes, leave residuals that distort the low-power authentication tag, degrading detection performance and elevating vulnerability to adversarial extraction.

Existing frameworks directly embed uncoded tags, resulting in:

• Robustness degradation under user interference: Even with perfect message recovery, subtraction yields estimated tags still corrupted by residual multiuser interference.
• Eavesdropping vulnerability: Adversaries can easily isolated the raw tag due to its direct embedding.

The paper’s thesis is that optimizing tag design—shifting from uncoded to frozen/coded tags—enables robust authentication and obfuscates tag extraction.

Frozen-Tag Framework and Modules

The proposed framework introduces two central innovations:

1. Frozen Tag Generation (FTG): Polar codes encode anchor information (message bits) as information bits and raw tags as frozen bits. The frozen tag is then sparsely and randomly embedded within the message, under key-driven position selection via hash functions.
2. Thaw Tag Reconciliation (TTR): At the receiver, the TTR module utilizes the same estimated raw tag as frozen bits for polar decoding, enabling error-resilient recovery of the anchor information.

Sparse Index Extraction (SIE) modules at both ends orchestrate random index selection and retrieval, further obfuscating the tag positions.

Security Advantage

Encoding the raw tag within the frozen bits of a polar code prevents direct adversary access and impedes brute-force attempts at tag recovery. Even if an eavesdropper guesses tag positions, estimating the content is hampered by accumulated noise due to required linear transformations.

Performance Analysis

Robustness

Authentication robustness is quantifiable via union bounds on detection probability, rooted in channel polarization properties. By allocating anchor information to high-reliability subchannels, the system maintains detection performance even under significant user interference. Numerical simulations show that increasing the length of the frozen tag improves detection probability and allows the authentication threshold to be set at the anchor information length, exploiting polar code characteristics.

Security

Security is analyzed against eavesdropping and spoofing:

• Position Confusion Challenge: Eve must identify tag positions without the secret key, facing both false alarm and miss detection probabilities determined by SNR and tag/message polarity relationships. Even at high SNR, $\text{P}_\text{err}$ remains nonzero, converging to an asymptotic value determined by the tag-to-message ratio, and the probability of accurate position classification diminishes exponentially with tag length.
• Tag Confusion Challenge: Even if tag positions are guessed, decoding the raw tag via pseudo-inverse mapping of the generator matrix accumulates substantial noise—average and maximum noise powers consistently exceed receiver noise, especially as tag length increases.
• Spoofing: The position and content of inserted tags for Eve are random without the key, and alignment probabilities between Eve’s and Alice’s positions decrease with message length; only a vanishing fraction of tag positions overlap, reducing the efficacy of adversarial spoofing.

Compatibility

To ensure communication integrity for unconscious receivers, the tag insertion operation is modeled as a cascaded BSC+AWGN channel. Upper bounds on BER are computed via Bhattacharyya parameters, demonstrating that, as message length increases or tag ratio decreases, BER approaches standard polar-coded transmission. This provides practical guidance for tag insertion ratio settings.

Experimental Results

• The proposed framework consistently outperforms uncoded tag-based baselines under multiuser interference, showing detection probability improvements of several dB in SINR.
• Numerical evaluation of security metrics demonstrates that Eve’s average bit error probability for tag position detection remains substantial at all SNRs, and probability of correct full classification is negligible for realistic tag lengths.
• Noise accumulation in raw tag estimation by Eve scales rapidly with frozen tag length, and overlap between legitimate and adversarial tag positions becomes insignificant as message length grows.
• Message BER for unconscious receivers is only marginally impacted for reasonable tag ratios, confirming high system compatibility.

Implications and Future Directions

By integrating polar-code-based frozen tag encoding and sparse embedding, this work establishes a robust, secure, and low-overhead PLA architecture for dense wireless environments. Theoretical bounds and numerical results both validate substantial improvements in detection, resilience, and secrecy relative to conventional tag-based approaches. The modular framework is readily extensible for hybrid authentication schemes and can be instantiated with other coding techniques (e.g., LDPC, Turbo) where adaptation is warranted.

The practical implication is clear: frozen-tag schemes enable lightweight, information-theoretic authentication suitable for real-time, multiuser-dense environments, addressing a crucial requirement for next-generation IoT, IIoT, and vehicular networks. Future work should focus on further complexity reduction, adaptive coding selection, and integration with challenge-response mechanisms to bolster robustness against evolving adversarial strategies in heterogeneous 6G ecosystems.

Conclusion

The frozen-tag-based PLA framework presented in the paper addresses fundamental vulnerabilities in traditional uncoded tag schemes by introducing robust tag encoding and obfuscation mechanisms. By employing polar-coded frozen tags and randomized position selection, the proposed approach significantly improves authentication performance under multiuser interference and fortifies security against eavesdropping and spoofing attacks, with minimal impact on compatibility. Theoretical analysis and simulation substantiate the claims, positioning frozen-tag PLA as a viable solution for secure, scalable, and efficient authentication in high-density wireless systems (2604.06641).