C2PA Metadata Location in AVIF ISOBMFF Containers

This article explains the physical location and structural mapping of Coalition for Content Provenance and Authenticity (C2PA) metadata within an AVIF file. AVIF is built on the ISO Base Media File Format (ISOBMFF), which organizes data into hierarchical structures called boxes or atoms. In an AVIF container, C2PA manifest stores are stored as dedicated metadata items defined inside the meta box, physically located within the file's payload data (typically the mdat box), and linked to the primary image item using explicit item references.

The ISOBMFF Architecture for C2PA

AVIF does not store arbitrary metadata in loose header fields. Instead, it relies on the standardized ISOBMFF item framework under the top-level meta box. The C2PA specification prescribes that provenance manifests must be packaged as a JUMBF (JPEG Universal Metadata Box Format) container and registered as an independent item in the ISOBMFF structural hierarchy.

1. Item Declaration (iinf and infe)

To declare the presence of C2PA metadata, the container uses the Item Information Box (iinf), located inside the root meta box:

• Inside iinf, there is an Item Information Entry (infe) box assigned specifically to the C2PA manifest.
• The item_type field in this infe entry is set to the FourCC (four-character code) 'c2pa'.
• The entry assigns a unique item_ID to represent this specific provenance manifest item throughout the container.

2. Physical Data Storage (iloc and mdat)

The binary content of the C2PA manifest—which consists of signed assertions, claims, and the manifest store—is physically stored in the media data space:

• Physical Payload: The actual raw bytes of the C2PA manifest are written directly inside the mdat (Media Data) box alongside the compressed AV1 image data.
• Byte Offset Mapping: The Item Location Box (iloc), located within the meta box, maps the C2PA item_ID to its exact physical location. The iloc specifies the base offset, extent offset, and byte length within the file where the reader can extract the raw C2PA payload.

3. Associating Metadata with the Image (iref)

For a reader to know which visual asset the C2PA metadata applies to, an explicit association is created inside the Item Reference Box (iref):

• An item reference entry is added with the reference type set to 'cdsc' (content describes).
• The from_item_ID points to the C2PA metadata item.
• The to_item_ID points to the primary visual item (the AV1 image item ID designated by the pitm box).

Alternative Placement: The uuid Box

While the item-based method using meta, infe, and iloc is the required standard for still image AVIF containers, ISOBMFF video tracks or non-item configurations may alternative encapsulate C2PA manifests inside a top-level uuid box. For AVIF still images, however, conforming implementations look exclusively inside the meta box item collection for an item of type 'c2pa'.