Permute-and-Flip: An optimally robust and watermarkable decoder for LLMs
Xuandong Zhao Lei Li Yu-Xiang Wang
In this paper, we propose a new decoding method called Permute-and-Flip (PF)
decoder. It enjoys robustness properties similar to the standard sampling
decoder, but is provably up to 2x better in its quality-robustness tradeoff
than sampling and never worse than any other decoder. We also design a
cryptographic watermarking scheme analogous to Aaronson's Gumbel watermark, but
naturally tailored for PF decoder. The watermarking scheme does not change the
distribution to sample, while allowing arbitrarily low false positive rate and
high recall whenever the generated text has high entropy. Our experiments show
that the PF decoder (and its watermarked counterpart) significantly
outperform(s) naive sampling (and it's Gumbel watermarked counterpart) in terms
of perplexity, while retaining the same robustness (and detectability), hence
making it a promising new approach for LLM decoding. The code is available at
https://github.com/XuandongZhao/pf-decoding