Here’s a fast, practical idea to speed up container scans: add a **hash‑based SBOM layer cache** keyed by **(Docker layer digest + dependency‑manifest checksum)** so identical inputs skip recomputation and only verify attestations.

---

### What this is (in plain words)

* **Layers are immutable.** Each image layer already has a content digest (e.g., `sha256:...`).
* **Dependency state is declarative.** Lockfiles/manifest files (NuGet `packages.lock.json`, `package-lock.json`, `poetry.lock`, `go.sum`, etc.) summarize deps.
* If both the **layer bytes** and the **manifest content** are identical to something we’ve scanned before, recomputing the SBOM/VEX is wasted work. We can **reuse** the previous result (plus a quick signature/attestation check).

---

### Cache key

```
CacheKey = SHA256(
  concat(
    LayerDigestCanonical,          // e.g., "sha256:abcd..."
    '\n',
    ManifestAlgo,                  // e.g., "sha256"
    ':',
    ManifestChecksum               // hash of lockfile(s) inside the layer FS view
  )
)
```

* Optionally include toolchain IDs to prevent cross‑version skew:

  * `SbomerVersion`, `ScannerRulesetVersion`, `FeedsSnapshotId` (OSV/NVD feed epoch), `PolicyBundleHash`.

---

### When it hits

* **Exact same layer + same manifests** → return cached **SBOM component graph + vuln findings + VEX** and **re‑verify** the **DSSE/in‑toto attestation** and timestamps (freshness SLA).
* **Same layer, manifests absent** → fall back to byte‑level heuristics (package index cache); lower confidence.

---

### Minimal .NET 10 sketch (Stella Ops)

```csharp
public sealed record LayerInput(
    string LayerDigest,            // "sha256:..."
    string? ManifestAlgo,          // "sha256"
    string? ManifestChecksum,      // hex
    string SbomerVersion,
    string RulesetVersion,
    string FeedsSnapshotId,
    string PolicyBundleHash);

public static string ComputeCacheKey(LayerInput x)
{
    var s = string.Join("\n", new[]{
        x.LayerDigest,
        x.ManifestAlgo ?? "",
        x.ManifestChecksum ?? "",
        x.SbomerVersion,
        x.RulesetVersion,
        x.FeedsSnapshotId,
        x.PolicyBundleHash
    });
    using var sha = System.Security.Cryptography.SHA256.Create();
    return Convert.ToHexString(sha.ComputeHash(System.Text.Encoding.UTF8.GetBytes(s)));
}

public sealed class SbomCacheEntry
{
    public required string CacheKey { get; init; }
    public required byte[] CycloneDxJson { get; init; }        // gz if large
    public required byte[] VexJson { get; init; }
    public required byte[] AttestationDsse { get; init; }      // for re-verify
    public required DateTimeOffset ProducedAt { get; init; }
    public required string FeedsSnapshotId { get; init; }      // provenance
}
```

---

### Cache flow (Scanner)

1. **Before scan**

   * Extract manifest files from the union FS of the current layer.
   * Hash them (stable newline normalization).
   * Build `LayerInput`; compute `CacheKey`.
   * **Lookup** in `ISbomCache.Get(CacheKey)`.
2. **Hit**

   * **Verify attestation** (keys/policy), **check feed epoch** still within tolerance, **re‑sign freshness** if policy allows.
   * Emit cached SBOM/VEX downstream; mark provenance as “replayed”.
3. **Miss**

   * Run normal analyzers → SBOM → vuln match → VEX lattice.
   * Create **in‑toto/DSSE attestation**.
   * Store `SbomCacheEntry` and **index by**:

     * `CacheKey` (primary),
     * `LayerDigest` (secondary),
     * `(ecosystem, manifestChecksum)` for diagnostics.
4. **Invalidation**

   * Roll cache on **FeedsSnapshotId** bumps or **RulesetVersion** change.
   * TTL optional for emergency revocations; keep **attestation+provenance** for audit.

---

### Storage options

* **Local**: content‑addressed dir (`/var/lib/stellaops/sbom-cache/aa/bb/<cacheKey>.cjson.gz`).
* **Remote**: Redis or Mongo (GridFS) keyed by `cacheKey`; attach indexes on `LayerDigest`, `FeedsSnapshotId`.
* **OCI artifact**: push SBOM/VEX as OCI refs tied to layer digest (helps multi‑node CI).

---

### Attestation verification (quick)

* On hit: `Verify(AttestationDsse, Policy)`; ensure `subject.digest == LayerDigest` and metadata (`FeedsSnapshotId`, tool versions) matches required policy.
* Optional **freshness stamp**: a tiny, fast “verification attestation” you produce at replay time.

---

### Edge cases

* **Multi‑manifest layers** (polyglot): combine checksums in a stable order (e.g., `SHA256(man1 + '\n' + man2 + ...)`).
* **Runtime‑only diffs** (no manifest change): include **package index snapshot hash** if you maintain one.
* **Reproducibility drift**: include analyzer version & configuration knobs in the key so the cache never masks rule changes.

---

### Why this helps

* Cold scans compute once; subsequent builds (same base image + same lockfiles) **skip minutes of work**.
* Reproducibility becomes **measurable**: cache hit ratio per repo, per base image, per feed epoch.

---

### Quick tasks to add to Stella Ops

* [ ] Implement `LayerInput` + keying in `Scanner.WebService`.
* [ ] Add **Manifest Harvester** step per ecosystem (NuGet, npm, pip/poetry, go, Cargo).
* [ ] Add `ISbomCache` (local + Mongo/OCI backends) with metrics.
* [ ] Wire **attestation re‑verify** path on hits.
* [ ] Ship a **cache report**: hit/miss, time saved, reasons for miss (ruleset/feeds changed, manifest changed, new analyzer).

If you want, I can draft the actual C# interfaces (cache backend + verifier) and a tiny integration for your existing `Sbomer`/`Vexer` services next.