flex-auth/workplans/FLEX-WP-0024-decision-envelope-authenticity.md

id	type	title	domain	repo	status	owner	topic_slug	planning_priority	planning_order	depends_on_workplans	related_workplans	created	updated
FLEX-WP-0024	workplan	Sign the decision envelope: the response channel is unauthenticated	infotech	flex-auth	ready	claude	netkingdom	P1	240	FLEX-WP-0021	FLEX-WP-0019 FLEX-WP-0023	2026-09-06	2026-09-06

FLEX-WP-0024 — Sign the decision envelope: the response channel is unauthenticated

Opened by FLEX-DEC-2026-010, from a question secrets-engine asked rather than answering locally: how is the response channel authenticated? They declined to choose a transport control for flex-auth's service on the grounds that it is not a consumer's call. Correct, and it is why this is a flex-auth workplan and not a mitigation in their repo.

The stance today: pins serve plain HTTP, flex-auth.decision-record.v1 carries no signature, and a responder that knows the package id and version — both published — can return a well-formed effect: allow that passes every check a consumer performs.

The two things not to get backwards

The digests do not authenticate the responder and cannot. Every input to them is either sent by the caller or published, so a forger reproduces request_digest, policy_package_digest, and registry_snapshot_digest exactly. Publishing more digests makes a forged envelope look more authenticated, not less.

Fail-closed does not cover this. It protects against a PDP that is absent, not against one that lies. An unreachable PDP denies; a lying PDP allows.

1. Publish the stance and stop publishing bare Service names

id: FLEX-WP-0024-T01
status: done
priority: high

Owner: flex-auth.

Done 2026-09-06. docs/decision-record-contract.md states that digest recomputation is not responder verification, because the contract is the only document a consumer is guaranteed to read. docs/operator-caller-access-path.md carries the reproduced DNS probe and the channel analysis. The trailing-dot FQDN replaced the bare Service name in examples/secrets-engine/README.md, SCOPE.md, and the FLEX-WP-0021-T05 handover note — a bare *.svc.cluster.local name resolves through search ad.binect.de to one unrelated public host, and a name for a service that does not exist resolves to the same address.

2. Choose the signature shape and key custody

id: FLEX-WP-0024-T02
status: todo
priority: high

Owner: flex-auth; key custody is not ours to invent.

• Detached signature over the canonical envelope, preferred over transport-only mTLS for two reasons: it survives the transport, so a decision audited later is still attributable, where mTLS authenticates a connection that no longer exists; and it composes with FLEX-DEC-2026-009, since a record that names its authenticated caller and carries flex-auth's signature is an audit artifact where either alone is a claim.
• Decide the canonical form to sign. The existing canonical-JSON rules from docs/canonical-request-digest.md are the obvious basis, but the signed material is the envelope, not the request, and it must exclude the signature field itself. Write it down before implementing: an implicit canonicalisation rule is what FLEX-DEC-2026-007 had to correct.
• Do not mint a key in this repo. Route custody via warden route find "decision envelope signing key" and OpenBao, per the estate credential-routing rule. flex-auth decides the shape; it does not own key material.
• State key rotation and how a consumer discovers the current public key, since a consumer that cannot rotate trust has a pinned key that will one day be wrong.

Gate: the canonical form, the algorithm, and the custody path are written down and reviewed before any signing code exists.

3. Implement signing and verification

id: FLEX-WP-0024-T03
status: wait
priority: high

Owner: flex-auth.

• Add the signature to schemas/decision_envelope.schema.json as additive, and state in the contract that request_digest is unaffected so no consumer re-pins — the same note that was needed for v2 and for approval_binding_digest.
• Emit unsigned when no key is configured, and say so in the envelope rather than omitting the field silently. Absence stated, not implied — the pdp_digest precedent.
• Ship a verification example and a fixture pair in examples/secrets-engine/replay/: a genuine signed envelope and one whose payload was altered after signing, so a consumer's verifier is tested against a real failure rather than only a real success. A verifier that has never seen an invalid signature is untested, which is the lesson from every fixture suite in FLEX-DEC-2026-008.

Gate: an altered envelope fails verification, an unaltered one passes, and a consumer can perform both from the fixtures alone.

4. Report the gap to gate-house

id: FLEX-WP-0024-T04
status: wait
priority: medium

Owner: flex-auth.

§6.4 obligates the PEP to act on a decision and §9.3 splits degraded mode by owner, but nothing in the standard requires a PDP's response to be authenticable. So a fully conformant PEP can trust a forged allow while meeting every published obligation. That is a gap in the standard, not only in this implementation, and it belongs in the outstanding v0.8 assent round alongside the FLEX-DEC-2026-009 finding.