Here’s a simple, big‑picture primer on how a modern, verifiable supply‑chain security platform fits together—and what each part does—before we get into the practical wiring and artifacts.

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# Topology & trust boundaries (plain‑English)

Think of the system as four layers, each with a clear job and a cryptographic handshake between them:

1. **Edge** (where users & CI/CD touch the system)

* **StellaRouter / UI** receive requests, authenticate users/agents (OAuth2/OIDC), and fan them into the control plane.
* Trust boundary: everything from the outside must present signed credentials/attestations before it’s allowed deeper.

2. **Control Plane** (brains & policy)

* **Scheduler**: queues and routes work (scan this image, verify that build, recompute reachability, etc.).
* **Policy Engine**: evaluates SBOMs, VEX, and signals against policies (“ship/block/defer”) and produces **signed, replayable verdicts**.
* **Authority**: key custody & identity (who can sign what).
* **Attestor**: issues DSSE/in‑toto attestations for scans, verdicts, and exports.
* **Timeline / Notify**: immutable audit log + notifications.
* Trust boundary: only evidence and identities blessed here can influence decisions.

3. **Evidence Plane** (facts, not opinions)

* **Sbomer**: builds SBOMs from images/binaries/source (CycloneDX 1.6 / SPDX 3.0.1).
* **Excititor**: runs scanners/executors (code, binary, OS, language deps, “what’s installed” on hosts).
* **Concelier**: correlates advisories, VEX claims, reachability, EPSS, exploit telemetry.
* **Reachability / Signals**: computes “is the vulnerable code actually reachable here?” plus runtime/infra signals.
* Trust boundary: raw evidence is tamper‑evident and separately signed; opinions live in policy/verdicts, not here.

4. **Data Plane** (do the heavy lifting)

* Horizontal workers/scanners that pull tasks, do the compute, and emit artifacts and attestations.
* Trust boundary: workers are isolated per tenant; outputs are always tied to inputs via cryptographic subjects.

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# Artifact association & tenant isolation (why OCI referrers matter)

* Every image/artifact becomes a **subject** in the registry.
* SBOMs, VEX, reachability slices, and verdicts are published as **OCI referrers** that point back to that subject (no guessing or loose coupling).
* This lets you attach **multiple, versioned, signed facts** to the same build without altering the image itself.
* Tenants stay cryptographically separate: different keys, different trust roots, different namespaces.

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# Interfaces, dataflows & provenance hooks (what flows where)

* **Workers emit**:

  * **SBOMs** in CycloneDX 1.6 and/or SPDX 3.0.1.
  * **VEX claims** (affected/not‑affected, under‑investigation, fixed).
  * **Reachability subgraphs** (the minimal “slice” proving a vuln is or isn’t callable in this build).
  * All wrapped as **DSSE/in‑toto attestations** and **attached via OCI referrers** to the image digest.
* **Policy Engine**:

  * Ingests SBOM/VEX/reachability/signals, applies rules, and emits a **signed verdict** (OCI‑attached).
  * Verdicts are **replayable**: same inputs → same output, with the exact inputs hashed and referenced.
* **Timeline**:

  * Stores an **audit‑ready record** of who ran what, with which inputs, producing which attestations and verdicts.

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# Why this design helps in real life

* **Audits become trivial**: point an auditor at the image digest; they can fetch all linked SBOMs/VEX/attestations/verdicts and replay the decision.
* **Noise collapses**: reachability + VEX + policy means you block only what matters for *this* build in *this* environment.
* **Multi‑tenant safety**: each customer’s artifacts and keys are isolated; strong boundaries reduce blast radius.
* **No vendor lock‑in**: OCI referrers and open schemas (CycloneDX/SPDX/in‑toto/DSSE) let you interoperate.

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# Minimal “starter” policy you can adopt Day‑1

* **Gate** on any CVE with reachability=“reachable” AND severity ≥ High, unless a trusted VEX source says “not affected” with required evidence hooks (e.g., feature flag off, code path pruned).
* **Fail on unknowns** above a threshold (e.g., >N packages with missing metadata).
* **Require** signed SBOM + signed verdict for prod deploys; store both in Timeline.

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# Quick glossary

* **SBOM**: Software Bill of Materials (what’s inside).
* **VEX**: Vulnerability Exploitability eXchange (is a CVE actually relevant?).
* **Reachability**: graph proof that vulnerable code is (not) callable.
* **DSSE / in‑toto**: standardized ways to sign and describe supply‑chain steps and their outputs.
* **OCI referrers**: a registry mechanism to hang related artifacts (SBOMs, attestations, verdicts) off an image digest.

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# A tiny wiring sketch

```
User/CI → Router/UI → Scheduler ─→ Workers (Sbomer/Excititor)
                          │             │
                          │             └─→ emit SBOM/VEX/reachability (DSSE, OCI-referrers)
                          │
                   Policy Engine ──→ signed verdict (OCI-referrer)
                          │
                 Timeline/Notify (immutable audit, alerts)
```

If you want, I can turn this into a one‑pager architecture card, plus a checklist your PMs/engineers can use to validate each trust boundary and artifact flow in your Stella Ops setup.