fix(hmac): add per-request nonce to prevent replay attacks#24
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Extends the HMAC signature scheme to include a nonce (16 random bytes,
hex-encoded) in every request. The new message formula is:
HMAC-SHA256(secret, "{timestamp}:{nonce}:{pathname}")
The nonce is sent in a new `pmux-nonce` header and validated for correct
format (32 lowercase hex chars). This eliminates the replay window that
existed in v1 where any request could be replayed within the ±60s
clock-skew tolerance.
Full replay prevention via server-side nonce uniqueness enforcement
requires an HMAC_NONCES KV binding; that is tracked as a follow-up.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Terry Howe <terrylhowe@gmail.com>
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rbonestell
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May 26, 2026
…gents) The nonce PR (#24) inadvertently changed validateClientSignature (legacy unversioned paths) to also require the pmux-nonce header, breaking agents distributed via Homebrew that were built before nonce support was added. Restore validateClientSignature to the original formula: HMAC-SHA256(secret, "{timestamp}:{pathname}") Only validateClientSignatureV1 (/v1/ paths) requires the nonce: HMAC-SHA256(secret, "{timestamp}:{nonce}:{pathname}") This preserves backward compatibility for older agents using legacy paths while keeping replay-attack protection for current agents on /v1/ paths. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Summary
pmux-nonceheader{timestamp}:{pathname}(v1) to{timestamp}:{nonce}:{pathname}(v2), eliminating the replay window within the ±60s clock-skew toleranceDetails
Breaking change: clients must now send
pmux-noncealongsidepmux-signatureandpmux-timestamp. Requests missing this header receive401 { error: "missing client signature" }.Follow-up: Full replay prevention (server-side nonce uniqueness enforcement) requires an
HMAC_NONCESKV namespace binding. Nonce format is validated but uniqueness is not yet enforced — this is documented inhmac.tsand tracked as a follow-up.This PR is coordinated with TerryHowe/pmux-agent#fix/hmac-nonce which updates the agent (Go) side to generate and send the nonce.
Test plan
npm test)rejects when pmux-nonce header is missingrejects a nonce that is not 32 lowercase hex charsrejects a nonce with uppercase hex chars🤖 Generated with Claude Code