提出无损多比特水印框架，可长期可靠嵌入文本标识
MC$^2$Mark: Distortion-Free Multi-Bit Watermarking for Long Messages
- 通过多通道彩色重加权编码信息，保持生成文本分布不变
- 长消息水印检测准确率较次优方法提升近30%
- 适合需要高保真度和强溯源能力的生成文本场景
大型语言模型生成的文本已接近人类写作水平，亟需可靠的来源追溯手段。多比特水印可将标识嵌入生成文本，但现有方法难以兼顾文本质量与水印强度，尤其在长消息场景下表现不佳。本文提出MC²Mark，一种无损多比特水印框架，专为长消息的可靠嵌入与解码设计。核心思路是多通道彩色重加权：通过结构化分词重加权编码比特信息，同时保持分词分布无偏；结合多层序列重加权增强水印信号，并采用证据累积检测器实现消息恢复。实验表明，相较于已有方法，MC²Mark在可检测性与鲁棒性方面均有提升，短消息检测准确率近乎完美，长消息检测准确率超过次优方法近30%。
原文摘要 · Abstract (English)
Large language models now produce text indistinguishable from human writing, which increases the need for reliable provenance tracing. Multi-bit watermarking can embed identifiers into generated text, but existing methods struggle to keep both text quality and watermark strength while carrying long messages. We propose MC$^2$Mark, a distortion-free multi-bit watermarking framework designed for reliable embedding and decoding of long messages. Our key technical idea is Multi-Channel Colored Reweighting, which encodes bits through structured token reweighting while keeping the token distribution unbiased, together with Multi-Layer Sequential Reweighting to strengthen the watermark signal and an evidence-accumulation detector for message recovery. Experiments show that MC$^2$Mark improves detectability and robustness over prior multi-bit watermarking methods while preserving generation quality, achieving near-perfect accuracy for short messages and exceeding the second-best method by nearly 30% for long messages.
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