fix: verify the device's mutual-auth server proof (M8)#103
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Firmware computes CMAC(session_key, server_nonce||client_nonce||device_id) precisely so the client can authenticate the device, but authenticate() discarded the 16-byte proof returned by parse_authenticate_success(). A device (or MITM) that returned status OK without knowing the master key was accepted, deferring detection to the first encrypted exchange. Recompute the proof from the derived session key and constant-time-compare it (hmac.compare_digest), raising AuthenticationFailedError on mismatch before storing the session. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01JRrm95f1qNZzDM9r2SB6KW
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Summary
M8 — server proof never verified; mutual auth was one-way (🟠)
The firmware computes
CMAC(session_key, server_nonce ‖ client_nonce ‖ device_id)precisely so the client can authenticate the device. Butauthenticate()calledparse_authenticate_success()(which returns the 16-byte proof) and discarded the return value. A device — or a MITM — that returned status0x00without knowing the master key was accepted, deferring detection to the first encrypted exchange.Fix
Add
compute_server_proof(session_key, server_nonce, client_nonce, device_id)and, inauthenticate(), recompute the proof from the derived session key and constant-time-compare it (hmac.compare_digest), raisingAuthenticationFailedErroron mismatch before storing the session state.Test plan
uv run pytest -q→ 445 passed (2 new end-to-end tests: valid proof accepted, bogus proof rejected with no session stored)ruff,mypyclean🤖 Generated with Claude Code