The CNML bridge, sign + verify in the app

The CNML bridge, sign + verify in the app

In this chapter: an issued OIML certificate becomes a signed CNML document, and anyone verifies it, without leaving the app. Status: ● leg 1 shipped (TODO.integration/09; the CA chain + the transparency proofs are later legs).

---

1. What CNML is to the program

CNML (Certificat Numérique de Métrologie Légale) is the OIML SMART program’s machine-readable certificate format: an XML document per Recommendation, signed with XMLDSig (ECDSA P-256) and verifiable through a fixed check pipeline. It is implemented by the sibling cnml repo (renamed from digital-certificates) and consumed as a dependency, never copied (the bridge resolves the @cnml/* packages to their TypeScript sources; when they change, the bridge re-resolves).

2. The flow

our Certificate ─ project (the per-rec field map) → CNML cert JSON
                ─ validate against the per-rec schema FIRST
                  (a rejection lists every violation, signs nothing)
                ─ serialize (@cnml/cnml-xml) → CNML XML
                ─ sign (@cnml/cnml-crypto, the browser-held key)
                → signed CNML XML (XMLDSig enveloped, X.509 in KeyInfo)
                ─ verify (@cnml/cnml-crypto's check pipeline)
                → per-check verdict with reasons

The check pipeline, in order: xml-well-formed → schema-valid → signature → scope → crl → timestamp → transparency. skip is a first-class status (proofs absent ⇒ honest skip); warn is a documented honest state (a self-signed leg-1 cert carries no scope extension, the check’s own legacy doctrine).

3. The signing custody

The signing key is an ECDSA P-256 WebCrypto pair, generated and held in the browser (IndexedDB, the private key passphrase-wrapped , the CNML pattern: the signer is a person at a browser, not a server). The signature ALWAYS carries an X.509 certificate in KeyInfo, the embedded chain is what any verifier resolves without a trust store; when no chain is supplied, the bridge self-issues from the signing pair. The signed document + the signing record persist on the certificate’s json_data.cnml (evidence accrues, never rewrites, a re-sign appends to cnml_history).

4. The operator surface

The certificate detail page carries the CNML panel: Sign as CNML (the issuance gate’s final step, the certificate must be ACTIVE) and the verification panel (every check with its reason, a download of the .cnml.xml). A verify failure is never silent: the schema rejection lists every violation; a tamper or a swapped embedded certificate fails the signature check with its reason (both proven by test).

5. What leg 1 deliberately is NOT

• Not the CA: the PKI CA server issues real X.509 chains (leg 2); leg 1 signs with browser-held self-signed keys (the scope/CRL checks then honestly skip/warn).
• Not transparency onboarding: the OTS/tlog proofs embed when the infrastructure answers; the checks skip honestly until then.

6. Where the depth lives

The gated spec: docs/architecture/16-the-cnml-bridge.md (the oimlsmart/smart repository, member access today; the public mirror lands with the website wave), the field map table, the failure-semantics card catalogue, and the upstream fixes the bridge surfaced (the core schemas’ registration, the parse-shape normalization, the trusted-key path).

Next: The program config seam, OIML SMART as instance #1 of the app framework.