Added caBX chunk for Content Credentials #542

[svgeesus]

This intentionally brief chunk description does three things:

1. Says what it is for, largely drawing language from the Introduction of the Content Credentials spec
2. Justifies the chunk contents as "must be the same as C2PA CC in all other media", normatively references Content Credentials for the details
3. Justifies why a private chunk is being standardized

I updated the chunk ordering table to add caBX, requiring that it be before IDAT (CC suggests but does not require before IDAT) and also to disallow multiple caBX. @lrosenthol are those correct?

I have not yet updated the chunk ordering diagrams, pending review of this PR.

[ProgramMax]

LGTM. Awaiting @lrosenthol 's input.

[fintelia]

Should the document be repeating C2PA's security claims ("secure, tamper-evident ...") as-is? I've seen others dispute whether the chunk meets its security goals. For instance, this author has been quite vocal against it.

[svgeesus]

Thanks for the link, those should certainly be added to the Security Considerations.

Dropping an entire caBX is trivial, and adding a new one also seems simple; it depends ultimately on who signs what.

[svgeesus]

This analysis seems fairly worrying; in particular, a bunch of data was removed from a PNG image, other data added, and it still validates as unaltered.

[lrosenthol]

I'll review the PR in sec, but let me comment on those specific blogs...

The author of those pages has a long history of "intermixing" implementations with specifications. All of the issues in those blogs were in specific implementations that long been fixed! They also all pre-date the C2PA's new Conformance Program (https://c2pa.org/conformance/) and refer to older versions of our specification.

We can certainly point to the security sections of the C2PA specification itself if you wish.

[lrosenthol]

Just a few minor editorials

[lrosenthol]

Suggested change

	<p>The <span class="chunk">caBX</span> chunk contains content credentials
	<p>The <span class="chunk">caBX</span> chunk contains Content Credentials

[lrosenthol]

Suggested change

	in a secure, tamper-evident (cryptographically verifiable)
	in a tamper-evident (cryptographically verifiable)

[lrosenthol]

Suggested change

	to determine the authenticity of media.
	to determine the authenticity of media.
	It can also be used to declare whether a given image
	was created or edited by Generative AI or a human
	or a combination of same.

[lrosenthol]

Suggested change

	<a href="https://spec.c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_embedding_manifests_into_png">section A.3.2. of Content Credentials</a>
	<a href="https://spec.c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_embedding_manifests_into_png">section A.3.2. of the Content Credentials specification</a>