CNML format specification

This document specifies the CNML (Certificat Numerique de Metrologie Legale) digital certificate format.

Namespace and version

The CNML XML namespace is:

https://oimlsmart.org/schemas/cnml/1.0

https://oimlsmart.org/schemas/cnml/1.0

The version is encoded in the namespace URI. The current format version is 1.0.

Compatibility rule (W3C convention): same major version is compatible. Newer minor fields are gracefully ignored by older verifiers. Different major versions are incompatible.

Document structure

A CNML document is an XML document with the root element cnml:certificatNumeriqueMetrologieLegale. The document has two main sections:

1. administrativeData: certificate metadata, parties, recommendation
2. measurementResults: type evaluation results and characteristics

<cnml:certificatNumeriqueMetrologieLegale
    xmlns:cnml="https://oimlsmart.org/schemas/cnml/1.0"
    xmlns:unitsml="https://unitsml.org/ns/unitsml"
    xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
    schemaVersion="1.0">
  <cnml:administrativeData>...</cnml:administrativeData>
  <cnml:measurementResults>...</cnml:measurementResults>
</cnml:certificatNumeriqueMetrologieLegale>

<cnml:certificatNumeriqueMetrologieLegale
    xmlns:cnml="https://oimlsmart.org/schemas/cnml/1.0"
    xmlns:unitsml="https://unitsml.org/ns/unitsml"
    xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
    schemaVersion="1.0">
  <cnml:administrativeData>...</cnml:administrativeData>
  <cnml:measurementResults>...</cnml:measurementResults>
</cnml:certificatNumeriqueMetrologieLegale>

Per-Recommendation schemas

Each OIML Recommendation has its own JSON Schema that defines the fields specific to that instrument category. The schemas live at packages/cnml-schemas/src/schemas/R*.yaml.

The core certificate model (shared across all Recommendations) is defined in _core.yaml. Per-Recommendation schemas extend this model with Recommendation-specific fields.

Units

Measurement units use the UnitsML namespace. Unit identifiers resolve through UnitsDB, which traces back to BIPM Digital SI.

Signature

CNML documents are signed with XMLDSig (W3C XML Signature Syntax and Processing 1.1). The signature is enveloped inside the root element. See the XMLDSig profile for algorithm details.

Instance certificates

Instance certificates (tier 5) bind a specific physical instrument to its model certificate. The root element is cnml:instanceCertificate with tier="5".

References

• W3C XML Signature Syntax and Processing 1.1
• RFC 6962 Certificate Transparency
• OIML R-Recommendations (per-Recommendation schemas)