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arXiv · 2609.31767

Litter-Masked Block z-Channel: Designing Cover Distributions Against Public-Design Observers

Bin Han·Wenwen Chen·Yao Zhu·Rafael F. Schaefer·Giuseppe Caire·Hans D. Schotten
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Abstract

Networking defenses against traffic analysis fill idle slots with cover traffic, yet the physical layer carrying it still hands upper layers the erasure-only model of a silent-idle link, leaving the cover content unexploited. We shape that content: the idle-slot waveform is drawn from the codebook complement under a design distribution, the \emph{litter distribution}, chosen to defeat a public-design passive observer while the upper layer keeps its block z-channel model. The maximum a posteriori (MAP) receiver reduces to a single threshold on the log-likelihood ratio between the best codeword and the prior-weighted litter aggregate; its errors are silent corruptions the upper layer cannot detect and erasures that retransmission absorbs. Receiver design is thus one-dimensional, minimizing erasure under a cap on silent corruption, and the joint design a Stackelberg game against a Kullback-Leibler (KL) indistinguishability metric, solved by an alternating convex relaxation and corroborated by an independent reinforcement-learning policy. On a rate-$3/4$ $(16,12)$ parity-check (LDPC) code against an observer who knows her own channel but whose channel the transmitter knows only statistically, shaped litter cuts the observer's Stein detection exponent by $41$ to $48\%$ relative to uniform cover, flat over a $10$~dB span and in the activity rate. Below the point where the legitimate link turns reliable, no cover distribution admits an operating point; at that point shaping costs about one percent of throughput, and nothing measurable $2$~dB above it.

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Bin Han, Wenwen Chen, Yao Zhu, Rafael F. Schaefer, Giuseppe Caire, Hans D. Schotten. 2026-09-24. Litter-Masked Block z-Channel: Designing Cover Distributions Against Public-Design Observers. https://arxiv.org/abs/2609.31767

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