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arXiv · 2605.00348

Block-wise Codeword Embedding for Reliable Multi-bit Text Watermarking

Joeun Kim·HoEun Kim·Dongsup Jin·Young-Sik Kim
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Abstract

Recent multi-bit watermarking methods for large language models (LLMs) prioritize capacity over reliability, often conflating decoding with detection. Our analysis reveals that existing ECC-based extractors suffer from catastrophic false positive rates (FPR), and applying rejection thresholds merely collapses detection sensitivity (TPR) to random guessing. To resolve this structural limitation, we propose BREW (Block-wise Reliable Embedding for Watermarking), a framework shifting the paradigm to designated verification. BREW employs a two-stage mechanism: (i) blind message estimation via independent block voting, followed by (ii) window-shifting verification that rigorously validates the payload against local edits. Experiments demonstrate that BREW achieves a TPR of 0.965 with an FPR of 0.02 under 10% synonym substitution, demonstrating that the high-FPR issue is not an inherent trade-off of multi-bit watermarking, but a solvable structural flaw of prior decoding-centric designs. Our framework is model-agnostic and theoretically grounded, providing a scalable solution for reliable forensic deployment.

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Joeun Kim, HoEun Kim, Dongsup Jin, Young-Sik Kim. 2026-05-01. Block-wise Codeword Embedding for Reliable Multi-bit Text Watermarking. https://arxiv.org/abs/2605.00348

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