From TDMA to CDMA: A Multi-bit Watermark for Diffusion Language Models

AbstractWhile DLMs have emerged as an alternative to ARMs, robust content provenance mechanisms for this architecture remain unexplored. Existing multi-bit watermarking schemes, heavily reliant on the sequential context of ARMs, cannot be directly applied to DLMs. In this paper, we reframe the multi-bit watermarking problem through a novel Digital Signal Processing (DSP) lens. We draw an analogy between prior works and TDMA (Time Division Multiple Access) in telecommunications, revealing their inherent limitations. To overcome these limitations, we introduce CDMArk, the first multi-bit watermarking framework tailored for DLMs, orchestrating a paradigm shift from TDMA to CDMA (Code Division Multiple Access). Our method encodes the entire watermark message across all tokens holographically. We further provide rigorous statistical guarantees for the watermark detection process. Extensive experiments demonstrate that CDMArk achieves a new state-of-the-art Pareto frontier between imperceptibility and effectiveness.