fix(envelope): fail closed when an authenticated decrypt surfaces no skid (#52)#55
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…skid (#52) The authcrypt unpack branch trusted DataProofs' definitional invariant IsAuthenticated <=> non-empty skid without asserting it. Had it ever regressed, a null skid would silently skip the FR-CONSIST-01 from<->skid binding on an envelope still reported authenticated; an empty skid would escape as a raw ArgumentException off the FR-API-07 contract. Assert a surfaced skid whenever a decrypt is authenticated, rejecting with ConsistencyException — parity with the JWS branch's signer-kid guard (6f393e9). The check lives in AddressingConsistency as CheckAuthcryptSkidSurfaced so it is directly unit-tested; the EnvelopeReader call site is unreachable through today's parser by construction, so no current input changes behavior. Closes #52. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Closes #52.
What
Defense-in-depth guard in the authcrypt unpack branch: whenever a decrypt is authenticated,
EnvelopeReadernow asserts a non-emptyskidand rejects withConsistencyExceptionotherwise — parity with the JWS branch's empty-signer-kid guard (6f393e9).Why
The branch trusted DataProofs' definitional invariant
IsAuthenticated ⟺ non-empty skid(JweParser.cs:439/572) without asserting it. Both regression shapes were traced:from↔skidcheck is silently skipped;message.Fromwould be attacker-set on an envelope the whole stack reports as authenticated.CheckAuthcryptFromMatchesSkidthrows a rawArgumentException, escaping the FR-API-07 unpack exception contract (fails closed, wrong type).The guard covers both and fires before any trust state (
authenticated/senderKid) is set.Decisions (pre-made, per the issue)
ConsistencyException, as the issue specifies. The JWS branch'sCryptoExceptionasymmetry is deliberate and untouched — no scope creep into that branch, line-127 semantics, DataProofs, or the PRD.EnvelopeReadercall site is unreachable through the real parser by construction (the invariant is definitional upstream; no parser seam exists, deliberately). The check therefore lives inAddressingConsistencyasCheckAuthcryptSkidSurfacedand is unit-tested directly (null → throws, empty → throws, valid → passes), the same pattern as every other FR-CONSIST rule. Precedent: the JWS guard shipped without a message-level test.Risk
Zero behavior change for any input that exists today — no legitimate flow, no currently-craftable envelope reaches the guard. Full suite green: 757 passed, 0 failed (598 core + 159 interop), no existing test modified.
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