# Determinism Benchmark (cross-scanner) — Draft

Source: advisory “23-Nov-2025 - Benchmarking Determinism in Vulnerability Scoring”. This doc captures the runnable harness pattern and expected outputs for task BENCH-DETERMINISM-401-057.

## Goal
- Measure determinism rate, order-invariance, and CVSS delta σ across scanners when fed identical SBOM+VEX inputs with frozen feeds.

## Minimal harness (Python excerpt)
```python
# run_bench.py (excerpt) — deterministic JSON hashing
def canon(obj):
    return json.dumps(obj, sort_keys=True, separators=(',', ':')).encode()

def shas(b):
    return hashlib.sha256(b).hexdigest()

for sbom, vex in zip(SBOMS, VEXES):
    for scanner, tmpl in SCANNERS.items():
        for mode in ("canonical", "shuffled"):
            for i in range(10):
                if mode == "shuffled":
                    sb, vx = shuffle(sbom), shuffle(vex)
                out = run(tmpl.format(sbom=sb, vex=vx))
                norm = normalize(out)  # purl, vuln id, base_cvss, effective
                blob = canon({"scanner": scanner, "sbom": sbom,
                              "vex": vex, "findings": norm})
                results.append({
                    "hash": shas(blob), "mode": mode,
                    "run": i, "scanner": scanner, "sbom": sbom
                })
```

## Inputs
- 3–5 SBOMs (CycloneDX 1.6 / SPDX 3.0.1) + matching VEX docs covering affected/not_affected/fixed.
- Feeds bundle: vendor DBs (NVD, GHSA, OVAL) hashed and frozen.
- Policy: single normalization profile (e.g., prefer vendor scores, CVSS v3.1).

## Metrics
- Determinism rate = identical_hash_runs / total_runs (20 per scanner/SBOM).
- Order-invariance failures (# distinct hashes between canonical vs shuffled).
- CVSS delta σ vs reference; VEX stability (σ_after ≤ σ_before).

## Deliverables
- `bench/determinism/` with harness, hashed inputs, and `results.csv`.
- CI target `bench:determinism` producing determinism% and σ per scanner.

## Links
- Source advisory: `docs/product-advisories/23-Nov-2025 - Benchmarking Determinism in Vulnerability Scoring.md`
- Sprint task: BENCH-DETERMINISM-401-057 (SPRINT_0401_0001_0001_reachability_evidence_chain.md)