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

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.