SimKey: A Semantically Aware Key Module for Watermarking Language Models

• URL: http://arxiv.org/abs/2510.12828v2
• Date: Mon, 03 Nov 2025 18:20:37 GMT
• Title: SimKey: A Semantically Aware Key Module for Watermarking Language Models
• Authors: Shingo Kodama, Haya Diwan, Lucas Rosenblatt, R. Teal Witter, Niv Cohen,
• Abstract summary: We introduce SimKey, a semantic key module that strengthens watermark robustness.<n>SimKey uses locality-sensitive hashing over semantic embeddings to ensure that paraphrased text yields the same watermark key.<n>It improves watermark robustness to paraphrasing and translation while preventing harmful content from false attribution.
• Score: 19.115617392855768
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: The rapid spread of text generated by large language models (LLMs) makes it increasingly difficult to distinguish authentic human writing from machine output. Watermarking offers a promising solution: model owners can embed an imperceptible signal into generated text, marking its origin. Most leading approaches seed an LLM's next-token sampling with a pseudo-random key that can later be recovered to identify the text as machine-generated, while only minimally altering the model's output distribution. However, these methods suffer from two related issues: (i) watermarks are brittle to simple surface-level edits such as paraphrasing or reordering; and (ii) adversaries can append unrelated, potentially harmful text that inherits the watermark, risking reputational damage to model owners. To address these issues, we introduce SimKey, a semantic key module that strengthens watermark robustness by tying key generation to the meaning of prior context. SimKey uses locality-sensitive hashing over semantic embeddings to ensure that paraphrased text yields the same watermark key, while unrelated or semantically shifted text produces a different one. Integrated with state-of-the-art watermarking schemes, SimKey improves watermark robustness to paraphrasing and translation while preventing harmful content from false attribution, establishing semantic-aware keying as a practical and extensible watermarking direction.

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