Here’s a simple, practical way to think about a **SBOM‑first, VEX‑ready supply‑chain spine** and the **evidence graph + smart‑diff** you can build on top of it—starting from zero and ending with reproducible, signed decisions.

# SBOM‑first spine (VEX‑ready)

**Goal:** make the SBOM the canonical graph of “what’s inside,” then layer signed evidence (build, scans, policy) so every verdict is portable, replayable, and auditable across registries.

**Core choices**

* **Canonical graph:** treat **CycloneDX 1.6** and **SPDX 3.x** as first‑class. Keep both in sync; normalize component IDs (PURL/CPE), hashes, licenses, and relationships.
* **Attestations:** use **in‑toto + DSSE** for all lifecycle facts:

  * build (SLSA provenance),
  * scan results (vuln, secrets, IaC, reachability),
  * policy evaluation (allow/deny, risk budgets, exceptions).
* **Storage/transport:** publish everything as **OCI‑attached artifacts** via **OCI Referrers**:

  * `image:tag` → SBOM (spdx/cdx), VEX, SARIF, provenance, policy verdicts, exception notes—each a referrer with media type + signature.
* **Signatures:** cosign/sigstore (or your regional crypto: eIDAS/FIPS/GOST/SM) for **content‑addressed** blobs.

**Minimum viable workflow**

1. **Build step**

   * Produce identical SBOMs in CycloneDX and SPDX.
   * Emit SLSA‑style provenance attestation.
2. **Scan step(s)**

   * OS + language deps + container layers; add **reachability proofs** where possible.
   * Emit one **scan attestation per tool** (don’t conflate).
3. **Policy step**

   * Evaluate policies (e.g., OPA/Rego or your lattice rules) **against the SBOM graph + scan evidence**.
   * Emit a **signed policy verdict attestation** (pass/fail + reasons + unknowns count).
4. **Publish**

   * Push image, then push SBOMs, VEX, scan attestations, policy verdicts as **OCI referrers**.
5. **Verify / consume**

   * Pull the image’s **referrer set**; verify signatures; reconstruct graph locally; **replay** the policy evaluation deterministically.

**Data model tips**

* Stable identifiers: PURLs for packages, digests for layers, Build‑ID for binaries.
* Edges: `component→dependsOn`, `component→vulnerability`, `component→evidence(attestation)`, `component→policyClaim`.
* Keep **time (as‑of)** and **source** on every node/edge for replay.

# Evidence graph + smart‑diff

**Goal:** persist an **explainability graph** (findings ↔ components ↔ provenance ↔ policies) and compute **signed delta‑verdicts** on diffs to drive precise impact analysis and quiet noise.

**What to store**

* **Provenance:** who built it, from what, when (commit, builder, materials).
* **Findings:** CVEs, misconfigs, secrets, license flags, each with source tool, version, rule, confidence, timestamp.
* **Policies & verdicts:** rule set version, inputs’ hashes, outcome, rationale.
* **Reachability subgraphs:** the minimal path proving exploitability (e.g., symbol → function → package → process start).

**Smart‑diff algorithm (high level)**

* Compare two images (or SBOM graphs) **by component identity + version + hash**.
* For each change class:

  * **Added/removed/changed component**
  * **New/cleared/changed finding**
  * **Changed reachability path**
  * **Changed policy version/inputs**
* Re‑evaluate only affected subgraph; produce a **Delta Verdict**:

  * `status`: safer / risk‑equal / risk‑higher
  * `why`: list of net‑new reachable vulns, removed reachable vulns, policy/exception impacts
  * `evidenceRefs`: hashes of attestations used
* **Sign the delta verdict (DSSE)** and publish it as an **OCI referrer** too.

**UX essentials**

* Artifact page shows: **“Evidence Stack”** (SBOM, scans, VEX, policy, provenance) with green checks for signatures.
* **Smart‑diff view:** left vs right image → “net‑new reachable CVEs (+3)”, “downgraded risk (‑1)” with drill‑downs to the exact path/evidence.
* **Explain button:** expands to show **why** a CVE is (not) applicable (feature flag off, code path unreachable, kernel mitigation present, etc.).
* **Replay badge:** “Deterministic ✅” (inputs’ hashes match; verdict reproducible).

# Implementation checklist (team‑ready)

**Pipelines**

* [ ] Build: emit SBOM (CDX + SPDX), SLSA provenance (in‑toto/DSSE), sign all.
* [ ] Scan: OS + language + config + (optional) eBPF/runtime; one attestation per tool.
* [ ] Policy: evaluate rules → signed verdict attestation; include **unknowns count**.
* [ ] Publish: push all as OCI referrers; enable verification gate on pull/deploy.

**Schema & IDs**

* [ ] Normalize component IDs (PURL/CPE) + strong hashes; map binaries (Build‑ID → package).
* [ ] Evidence graph store: Postgres (authoritative) + cache (Valkey) for queries.
* [ ] Index by image digest; maintain **as‑of** snapshots for time‑travel.

**Determinism**

* [ ] Lock feeds, rule versions, tool versions; record all **input digests**.
* [ ] Provide a `replay.yaml` manifest capturing inputs → expected verdict hash.

**Security & sovereignty**

* [ ] Pluggable crypto: eIDAS/FIPS/GOST/SM; offline bundle export/import.
* [ ] Air‑gapped profile: Postgres‑only with documented trade‑offs.

**APIs & types (suggested media types)**

* `application/vnd.cyclonedx+json`
* `application/spdx+json`
* `application/vnd.in-toto+json; statement=provenance|scan|policy`
* `application/vnd.stella.verdict+json` (your signed verdict/delta)

**Minimal object examples (sketches)**

*Attestation (scan)*

```json
{
  "type": "https://in-toto.io/Statement/v1",
  "predicateType": "https://stella.dev/scan/v1",
  "subject": [{"name": "registry/app@sha256:…", "digest": {"sha256": "..."} }],
  "predicate": {
    "tool": {"name": "scannerX", "version": "1.4.2"},
    "inputs": {"sbom": "sha256:…", "db": "sha256:…"},
    "findings": [{"id": "CVE-2025-1234", "component": "pkg:pypi/xyz@1.2.3", "severity": "HIGH"}]
  }
}
```

*Policy verdict (replayable)*

```json
{
  "type": "https://in-toto.io/Statement/v1",
  "predicateType": "https://stella.dev/verdict/v1",
  "subject": [{"name": "registry/app@sha256:…"}],
  "predicate": {
    "policy": {"id": "prod.v1.7", "hash": "sha256:…"},
    "inputs": {"sbom": "sha256:…", "scans": ["sha256:…","sha256:…"]},
    "unknowns": 2,
    "decision": "allow", 
    "reasons": [
      "CVE-2025-1234 not reachable (path pruned)",
      "License policy ok"
    ]
  }
}
```

*Delta verdict (smart‑diff)*

```json
{
  "predicateType": "https://stella.dev/delta-verdict/v1",
  "predicate": {
    "from": "sha256:old", "to": "sha256:new",
    "impact": "risk-higher",
    "changes": {
      "componentsAdded": ["pkg:apk/openssl@3.2.1-r1"],
      "reachableVulnsAdded": ["CVE-2025-2222"]
    },
    "evidenceRefs": ["sha256:scanA", "sha256:policyV1"]
  }
}
```

# Operating rules you can adopt today

* **Everything is evidence.** If it influenced a decision, it’s an attestation you can sign and attach.
* **Same inputs → same verdict.** If not, treat it as a bug.
* **Unknowns budgeted by policy.** E.g., “fail prod if unknowns > 0; warn in dev.”
* **Diffs decide deployments.** Gate on the **delta verdict**, not raw CVE counts.
* **Portable by default.** If you move registries, your decisions move with the image via referrers.

If you want, I can turn this into starter repos (SBOM/attestation schemas, OCI‑referrer publish/verify CLI, and a smart‑diff service stub in .NET 10) so your team can plug it into your current pipelines without a big rewrite.