TextSeal2 kept its watermark intact under speculative decoding and detected 18 to 22 percentage points more long answers than its predecessor
AspireAI community · arXiv (Meta / FAIR)
Watermarking LLM output normally trades detection strength for output quality, and speculative decoding breaks the trade because rejected draft tokens get replaced by tokens the watermark never touched. TextSeal2 fixes this by merging two or more keys into a single virtual key and coupling it to fresh noise through AutoMark's Gaussian copula, so replaced tokens stay watermarked with the same keys and the acceptance rate is unchanged. Under speculative decoding it verifies keyed drafts with one extra exponential race run on the leftovers of the draft races, which by memorylessness reproduces block verification so the output still follows the target model exactly. At equal output diversity TextSeal2 detects 8 to 22 percentage points more 100-token answers than TextSeal at a false-positive rate of 10^-3, and under speculative decoding it detects 18 and 22 percentage points more 200-token answers than TextSeal and textGrain respectively. The paper reports no measurable inference latency overhead. Code is released in the Meta textseal repository, and the evaluation covers three LLMs at one stated false-positive rate.
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