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Speculative decoding leaked a private document to the cloud in rejected drafts, without ever showing it in the answer

AspireAI community · arXiv
Web assistants increasingly pair a small on-device model with a cloud LLM, sending only a public view of the user's question while the document stays local. This paper shows the message transcript can disclose private content that appears in neither the public view nor the answer: in a planted-secret audit, speculative decoding with private drafts never put the secret code in the answer but did send it to the cloud inside rejected drafts. ADLER is a decoding protocol for streaming LLM APIs in which every cloud message depends only on the public view and the answer prefix, with a client-side leakage ledger capping how much the answer itself reveals. Across the audit, no ADLER configuration exposed the secret beyond the answer. On GSM8K with the numbers masked from the cloud, ADLER lifted a 0.5B client's exact-match accuracy from 0.41 to 0.83 and a 1.5B client's from 0.67 to 0.85 under lenient parsing, against 0.92 for uploading the whole problem, and beat a zero-leakage program baseline of 0.69. The disclosure is priced in nats, an approximate currency, and the ledger bounds what the answer gives away rather than guaranteeing zero leakage.
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