With the development of large language models, multiple AIs are now made available for code generation (such as ChatGPT and StarCoder) and are adopted widely. It is often desirable to know whether a piece of code is generated by AI, and furthermore, which AI is the author. For instance, if a certain version of AI is known to generate vulnerable codes, it is particularly important to know the creator. Existing approaches are not satisfactory as watermarking codes are more challenging compared to watermarking text data, as codes can be altered with relative ease via widely-used code refactoring methods. In this work, we propose ACW (AI Code Watermarking), a novel method for watermarking AI-generated codes. The key idea of ACW is to selectively apply a set of carefully-designed semantic-preserving, idempotent code transformations, whose presence (or absence) allows us to determine the existence of the watermark. It is efficient as it requires no training or fine-tuning and works in a black-box manner. It is resilient as the watermark cannot be easily removed or tampered through common code refactoring methods. Our experimental results show that ACW is effective (i.e., achieving high accuracy, true positive rates and false positive rates) and resilient, significantly outperforming existing approaches.

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