docs: document zeroization scope and the HKDF/HMAC state limitation - #20
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nadimkobeissi merged 1 commit intoJul 24, 2026
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The README's security posture covered constant-time properties but not memory-hygiene guarantees. Add a Zeroization section stating what the crate scrubs, and disclose the one gap it cannot close: the RustCrypto hkdf/hmac crates do not zeroize internal HMAC state on drop, so PRK-derived ipad/opad block state transiently survives in freed memory after every extract/expand. Shared by all RustCrypto-based HPKE implementations; documented so deployments with a memory-forensics threat model can compensate (no core dumps, encrypted swap). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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It was unfair of me to reject this, sorry. |
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Summary
From a security review of v0.1.0: the crate's own zeroization discipline is excellent — private keys, shared secrets, PRKs, seeds, and the derived AEAD key/base nonce are all scrubbed, including manual scrubs where upstream types lack zeroize-on-drop. But one gap can't be closed from this crate, and it wasn't disclosed anywhere: the RustCrypto
hkdf/hmaccrates do not zeroize their internal HMAC state on drop. EachLabeledExtract/LabeledExpand(e.g. the discardedHkdfhalf ofHkdfExtract::finalize()insrc/kdf.rs, and the per-callHkdf::from_prk) leaves PRK-derived ipad/opad block state — key-equivalent material — transiently in freed memory.Since the README already carries a constant-time disclosure table, this adds the matching memory-hygiene disclosure: a Zeroization section stating what is scrubbed, the known limitation, why it's ecosystem-wide, and what deployments with a memory-forensics threat model should do about it (no core dumps, encrypted swap).
Documentation only; no code changes.
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