Metadata Assertion

The C2PA technical specification allows actors in a workflow to make cryptographically signed assertions about the produced C2PA asset.

This specification describes a C2PA assertion referred to here as the metadata assertion that can be added to a C2PA Manifest to allow metadata from various metadata standards (including, but not limited to, XMP, IPTC, and Exif) to be cryptographically bound to the C2PA Manifest.

This specification differs from the c2pa.metadata assertion from the C2PA technical specification in that there are no restrictions on the metadata fields which can be represented.

Version 1.0 Approved 18 March 2024 · Version history

Maintainers:

License

This specification is subject to the Community Specification License 1.0.

Additional information about this specification’s scope and governance can be found at the project’s GitHub repository (creator-assertions/metadata-assertion). The Community Specification License documents at the root of that repository are the authoritative governance documents for this specification.

Contributing

This section is non-normative.

This publicly available specification was approved by the Creator Assertions Working Group on 18 March 2024.

If you wish to contribute to the development of a future version of this specification, you are invited to:

Foreword

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. No party shall not be held responsible for identifying any or all such patent rights.

Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.

This document was prepared by the Creator Assertions Working Group.

Known patent licensing exclusions are available in the specification’s notices.md file.

Any feedback or questions on this document should be directed to the specifications repository (GitHub: creator-assertions/metadata-assertion).

THESE MATERIALS ARE PROVIDED “AS IS.” The Contributors and Licensees expressly disclaim any warranties (express, implied, or otherwise), including implied warranties of merchantability, non-infringement, fitness for a particular purpose, or title, related to the materials. The entire risk as to implementing or otherwise using the materials is assumed by the implementer and user. IN NO EVENT WILL THE CONTRIBUTORS OR LICENSEES BE LIABLE TO ANY OTHER PARTY FOR LOST PROFITS OR ANY FORM OF INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES OF ANY CHARACTER FROM ANY CAUSES OF ACTION OF ANY KIND WITH RESPECT TO THIS DELIVERABLE OR ITS GOVERNING AGREEMENT, WHETHER BASED ON BREACH OF CONTRACT, TORT (INCLUDING NEGLIGENCE), OR OTHERWISE, AND WHETHER OR NOT THE OTHER MEMBER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Table of contents

1. Introduction

This section is non-normative.

1.1. Scope

For purposes of the Community Specification License, the scope.md document at the root of this project’s GitHub repository is the governing document of this specification’s scope.

1.2. History

In versions 1.3 and earlier of the C2PA technical specification, there were individual assertions for each metadata standard (e.g., IPTC, EXIF). These were coalesced into a single “common” metadata assertion in version 1.4 of the specification. In version 2.0 of the C2PA technical specification, new restrictions were added as to which metadata content could be reflected in the built-in c2pa.metadata assertion.

This specification is not a product of the C2PA, but provides a mechanism for metadata that can no longer be expressed in the c2pa.metadata assertion to be included in a C2PA Manifest.

2. Normative references

3. Assertion definition

A CAWG metadata assertion SHALL have a label of cawg.metadata and SHALL follow the format for metadata assertions as described below:

Each metadata assertion shall contain a single JSON content type box containing the JSON-LD serialization of one or more metadata values. The @context property within the JSON-LD object shall be included, and used to provide context / namespaces for the metadata standards being specified. The recommended procedure to create this JSON-LD object is to first create an XMP Data Model representation of the metadata and then serialize that to JSON-LD using the JSON-LD serialization of XMP. The JSON-LD would then be stored as a JSON content type box.
— Section 18.14.1, “Metadata: Common Requirements,” of the C2PA technical specification version 2.0
The restrictions expressed in Appendix A, “Implementation Details for c2pa.metadata,” in the C2PA technical specification SHALL NOT apply to this assertion.

3.1. Examples

This section is non-normative.

An example of an common metadata assertion for an image:

{
	"@context" : {
		"exif": "http://ns.adobe.com/exif/1.0/",
		"exifEX": "http://cipa.jp/exif/2.32/",
		"tiff": "http://ns.adobe.com/tiff/1.0/",
		"Iptc4xmpCore": "http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/",
		"Iptc4xmpExt": "http://iptc.org/std/Iptc4xmpExt/2008-02-29/",
		"dc" : "http://purl.org/dc/elements/1.1/",
		"photoshop" : "http://ns.adobe.com/photoshop/1.0/",
	},
	"photoshop:DateCreated": "Aug 31, 2022", 
	"Iptc4xmpExt:DigitalSourceType": "https://cv.iptc.org/newscodes/digitalsourcetype/digitalCapture",
	"Iptc4xmpExt:LocationCreated": { 
	  "Iptc4xmpExt:City": "San Francisco"
	},
	"Iptc4xmpExt:PersonInImage": [ 
	  "Erika Fictional"
	],
	"Iptc4xmpCore:AltTextAccessibility": "Photo of Erika Fictional standing in front of the Golden Gate Bridge at sunset.",
	"exif:GPSVersionID": "2.2.0.0",
	"exif:GPSLatitude": "39,21.102N",
	"exif:GPSLongitude": "74,26.5737W",
	"exif:GPSAltitudeRef": 0,
	"exif:GPSAltitude": "100963/29890",
	"exif:GPSTimeStamp": "2019-09-22T18:22:57Z",
	"exif:GPSSpeedRef": "K",
	"exif:GPSSpeed": "4009/161323",
	"exif:GPSImgDirectionRef": "T",
	"exif:GPSImgDirection": "296140/911",
	"exif:GPSDestBearingRef": "T",
	"exif:GPSDestBearing": "296140/911",
	"exif:GPSHPositioningError": "13244/2207",
	"exif:ExposureTime": "1/100",
	"exif:FNumber": 4.0,
	"exif:ColorSpace": 1,
	"exif:DigitalZoomRatio": 2.0,
	"tiff:Make": "CameraCompany",
	"tiff:Model": "Shooter S1",
	"exifEX:LensMake": "CameraCompany",
	"exifEX:LensModel": "17.0-35.0 mm",
	"exifEX:LensSpecification": { "@list": [ 1.55, 4.2, 1.6, 2.4 ] }
  
}

An example of an common metadata assertion for a PDF:

{
	"@context" : {
	  "dc" : "http://purl.org/dc/elements/1.1/",
	  "xmp" : "http://ns.adobe.com/xap/1.0/",
	  "pdf" : "http://ns.adobe.com/pdf/1.3/",
	  "pdfx": "http://ns.adobe.com/pdfx/1.3/"
	},
	"dc:created": "2015 February 3", 
	"dc:title": [
		"This is a test file"
	],
	"xmp:CreatorTool": "TeX",
	"pdf:Producer": "pdfTeX-1.40.14",
	"pdf:Trapped": "Unknown",
	"pdfx:PTEX.Fullbanner": "This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013) kpathsea version 6.1.1"
}

Appendix A: Version history

This section is non-normative.

28 February 2024

04 March 2024

  • Promoted from pre-draft to draft status.

18 March 2024

  • Approved 1.0 version of this specification.