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Time Will Tell: Timing Side Channels via Output Token Count in Large Language Models

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arxiv 2412.15431 v1 pith:W2IBS2CB submitted 2024-12-19 cs.LG cs.CLcs.CR

Time Will Tell: Timing Side Channels via Output Token Count in Large Language Models

classification cs.LG cs.CLcs.CR
keywords outputtaskslanguagemodelsside-channeladversarycountllms
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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This paper demonstrates a new side-channel that enables an adversary to extract sensitive information about inference inputs in large language models (LLMs) based on the number of output tokens in the LLM response. We construct attacks using this side-channel in two common LLM tasks: recovering the target language in machine translation tasks and recovering the output class in classification tasks. In addition, due to the auto-regressive generation mechanism in LLMs, an adversary can recover the output token count reliably using a timing channel, even over the network against a popular closed-source commercial LLM. Our experiments show that an adversary can learn the output language in translation tasks with more than 75% precision across three different models (Tower, M2M100, MBart50). Using this side-channel, we also show the input class in text classification tasks can be leaked out with more than 70% precision from open-source LLMs like Llama-3.1, Llama-3.2, Gemma2, and production models like GPT-4o. Finally, we propose tokenizer-, system-, and prompt-based mitigations against the output token count side-channel.

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Cited by 4 Pith papers

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