feat(spec): Trust Manifest threat modeling review - suggested updates.

adr/0017-trust-manifest-artifact-binding.md

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+# ADR-0017: Bind Trust Manifest Signatures to Artifact Content
+
+**Status:** Accepted
+
+**Date:** 2026-06-20
+
+**Participants:** Luca Muscariello (Cisco)
+
+## Context
+
+[ADR-0009](0009-trust-manifest-substitution.md) acknowledged that Trust
+Manifest substitution is a real threat and called for a security review.
+That review was performed as a STRIDE threat model (see
+[trust-manifest-threat-model.md](../specification/trust-manifest-threat-model.md))
+and confirmed the central weakness: the `signature` covered only the
+Trust Manifest JSON, while the artifact reference (`url`, `data`,
+`mediaType`) lived on the Catalog Entry *outside* the signed payload.
+The only binding was `identity == identifier`. An attacker who controlled
+the catalog document could keep a validly-signed manifest and repoint
+`url` to a malicious artifact — the signature would still verify
+(threat T1 / finding F1).
+
+The threat model also compared the design against the Sigstore
+architecture from which it derives. Sigstore's Cosign signs the artifact
+*digest*, Fulcio binds identity through a CA, and Rekor witnesses the
+event in a transparency log. The Trust Manifest natively reproduced none
+of these; most importantly it lacked Cosign's digest binding.
+
+## Decision
+
+Bind a signed Trust Manifest to the artifact it describes and harden the
+verification story:
+
+1. **Subject binding.** A signed Trust Manifest MUST include a `subject`
+   object `{ url?, mediaType, digest }` committing to the artifact's
+   media type and content digest. `subject` is part of the signed
+   payload, so the artifact reference cannot be changed without
+   invalidating the signature. (Closes F1; mirrors Cosign.)
+2. **Signatures required at Level 3.** A Trusted Catalog MUST carry a
+   `signature`, `subject`, and `issuedAt` on each relied-upon entry, and
+   consumers MUST verify them. (Closes F2.)
+3. **Trust anchoring.** Verification MUST anchor `identity` to an
+   out-of-band trust root; a verified signature alone proves internal
+   consistency, not publisher authenticity. (Closes F3; substitutes for
+   Fulcio's CA-attested identity.)
+4. **Algorithm allowlist.** Consumers MUST reject `alg: none` and
+   symmetric algorithms and MUST use asymmetric JWS algorithms. (F4)
+5. **Freshness.** Add `issuedAt` (REQUIRED when signed) and `expiresAt`,
+   with anti-rollback and revocation guidance. (F5)
+6. **Catalog-level integrity.** Add an OPTIONAL top-level catalog
+   `signature` and recommend content-addressed (OCI) distribution. (F6)
+7. **Safe fetching.** Verification MUST defend against SSRF and
+   oversized responses. (F7)
+8. **Provenance-statement and publisher verification** procedures are
+   defined; publisher fields are advisory unless bound. (F8, F9)
+
+## Sigstore convergence
+
+The Trust Manifest remains a *format that carries* trust evidence, not a
+replacement for Sigstore services. The specification SHOULD allow
+Sigstore evidence to be first-class: a `sigstore-bundle` attestation
+(certificate + signature + Rekor inclusion proof), anchoring via the
+Sigstore TUF root, and a preference for transparency-log inclusion and
+keyless signing over long-lived publisher keys. This lets
+trust-sensitive deployments inherit Sigstore's full chain instead of a
+weaker re-implementation. Tracked as future work.
+
+## Consequences
+
+- The substitution attack from ADR-0009 is closed for signed manifests.
+- Level 3 conformance is stricter: presence of a manifest is no longer
+  sufficient; it must be signed and bound.
+- Residual gaps versus Sigstore remain (no native transparency log,
+  long-lived keys, trust-anchor bootstrapping) and are documented as
+  residual risks plus the Sigstore-convergence roadmap.

adr/0018-substantive-trust-manifest.md

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+# ADR-0018: Require a Substantive Trust Manifest
+
+**Status:** Accepted
+
+**Date:** 2026-06-21
+
+**Participants:** Luca Muscariello (Cisco)
+
+## Context
+
+The Trust Manifest is an OPTIONAL companion to a Catalog Entry. Within
+it, however, only `identity` was unconditionally REQUIRED. Every other member — `signature`,
+`attestations`, `provenance`, `trustSchema`, and the informational fields
+— was independently OPTIONAL.
+
+As a result the smallest valid Trust Manifest was `{ "identity": "..." }`,
+which carries no trust signal by itself.
+Only Level 3 (Trusted Catalog) forced a real payload (`signature` +
+`subject` + `issuedAt`). Between "no manifest" and Level 3 the format
+permitted a present-but-empty manifest that looks like trust metadata
+while asserting nothing — misleading to consumers and noise to tooling.
+
+If every field is optional, the unit of optionality is wrong: the
+*manifest* should be the optional element, and when an author chooses to
+include one it should be required to be meaningful.
+
+## Decision
+
+Define a validity floor for the Trust Manifest. Beyond the required
+`identity`, a Trust Manifest MUST contain at least one *substantive* trust
+member:
+
+- a `signature` (with its required `subject` and `issuedAt`),
+- a non-empty `attestations` array,
+- a non-empty `provenance` array, or
+- a `trustSchema`.
+
+`identity` and `identityType` (which identify the workload principal)
+and the informational members `privacyPolicyUrl`,
+`termsOfServiceUrl`, and `metadata` do not satisfy this requirement.
+`subject`, `issuedAt`, and `expiresAt` are not substantive on their own:
+an unsigned `subject` digest is attacker-settable and unverifiable, so
+they count only as part of a `signature`.
+
+A Trust Manifest that would carry only non-substantive members MUST be
+omitted entirely. Consumers SHOULD treat a manifest that violates this
+rule as if no Trust Manifest were present.
+
+## Consequences
+
+- A present Trust Manifest now always carries at least one verifiable or
+  governance-bearing claim; the degenerate identity-only manifest is no
+  longer valid.
+- Authors who have nothing substantive to assert simply omit
+  `trustManifest`, which is already OPTIONAL — no information is lost.
+- The rule is independent of conformance level: even a Level 1 catalog
+  that includes a manifest must make it substantive. Level 3 remains
+  stricter, requiring a `signature` specifically (see
+  [ADR-0017](0017-trust-manifest-artifact-binding.md)).
+- Unsigned manifests remain valid when they carry attestations,
+  provenance, or a trust schema, preserving low-assurance use cases that
+  do not sign.