feat: Add metadata assertion and support for validation

sdk/src/assertions/labels.rs

@@ -18,6 +18,9 @@
 //! These constants do not include version suffixes.
 //!
 //! See <https://c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_c2pa_standard_assertions>.
+use std::sync::LazyLock;
+
+use regex::Regex;
 
 /// Label prefix for a claim assertion.
 ///
@@ -184,9 +187,24 @@ pub(crate) const DATABOX_STORE: &str = "c2pa.databoxes";
 
 /// Label prefix for asset reference assertion.
 ///
-/// See <https://spec.c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_asset_reference>
+/// See <https://spec.c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_asset_reference>.
 pub const ASSET_REFERENCE: &str = "c2pa.asset-ref";
 
+/// Label prefix for a metadata assertion.
+///
+/// See <https://spec.c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_metadata>.
+pub const METADATA: &str = "c2pa.metadata";
+
+/// Must have a label that ends in '.metadata' and is preceded by an entity-specific namespace.
+/// For example, a 'com.litware.metadata' assertion would be valid.
+pub static METADATA_LABEL_REGEX: LazyLock<Regex> = LazyLock::new(|| {
+    #[allow(clippy::unwrap_used)]
+    {
+        Regex::new(r"^(?:[a-zA-Z0-9][a-zA-Z0-9_-]*)(?:\.(?:[a-zA-Z0-9][a-zA-Z0-9_-]*))*\.metadata$")
+            .unwrap()
+    }
+});
+
 /// Return the version suffix from an assertion label if it exists.
 ///
 /// When an assertion's schema is changed in a backwards-compatible manner,