Segmenting Watermarked Texts From Language Models

TL;DR

This paper introduces a statistical method to detect and segment watermarked segments within texts generated by language models, even when the text has been modified, ensuring source traceability.

Contribution

It proposes a novel change point detection approach to identify watermarked sub-strings in LLM-generated texts, handling modifications and ensuring error control.

Findings

Accurately detects watermarked segments in generated texts

Handles modifications like substitutions, insertions, deletions

Demonstrates effectiveness on texts from multiple language models

Abstract

Watermarking is a technique that involves embedding nearly unnoticeable statistical signals within generated content to help trace its source. This work focuses on a scenario where an untrusted third-party user sends prompts to a trusted language model (LLM) provider, who then generates a text from their LLM with a watermark. This setup makes it possible for a detector to later identify the source of the text if the user publishes it. The user can modify the generated text by substitutions, insertions, or deletions. Our objective is to develop a statistical method to detect if a published text is LLM-generated from the perspective of a detector. We further propose a methodology to segment the published text into watermarked and non-watermarked sub-strings. The proposed approach is built upon randomization tests and change point detection techniques. We demonstrate that our method ensures Type I and Type II error control and can accurately identify watermarked sub-strings by finding the corresponding change point locations. To validate our technique, we apply it to texts generated by several language models with prompts extracted from Google's C4 dataset and obtain encouraging numerical results. We release all code publicly at https://github.com/doccstat/llm-watermark-cpd.