On C2PA · ongoing by Tim Bray

This article caught my eye for three reasons: Leica, Digital Signatures, and Tim Bray.

Many have commented and worried about how we can trust any photos in the future now that LLM's can just create them out of any description. How will we ever be able to trust that the image we are looking at is actually a photograph and not just made up. Public Key cryptography does solve this problem using key pairs with signatures. This Leica M11-P has C2PA (Coalition for Content Provenance and Authenticity) to show a verified signature chain all the way from the capture on the sensor to the user looking at the photo.

When you take a picture, the camera makes a little data package called a "manifest", which records a bunch of useful stuff like the time, the camera serial number, the name of the person who owns the camera, and so on. Then it runs a bunch of math over the private key and manifest data and the image pixels to produce a little binary blob called the "signature"; the process is called "signing". The manifest and the signature are stored inside the metadata (called "EXIF") that every digital photo has.

You are probably wondering the same thing I am, what about post processing? The above is great but then I pull it into Photoshop and all hope is lost. Actually, Photoshop is working on support too.

Now, suppose you change the photo in Photoshop and save it. It turns out that Photoshop already has (a beta version of) C2PA built in , and your copy on your own computer has its own private/public keypair. So, first of all, it can verify the incoming photo's signature. Then when you save the edited version, Photoshop keeps the old C2PA manifest but also adds another, and uses its own private key to sign a combination of the new manifest, the old manifest (and its signature), and the output pixels.

There's enough information in there that if you have the public keys of my camera and my copy of Photoshop, you can verify that this was a photo from my camera that was processed with my Photoshop installation, and nobody else got in there to make any changes. Remember, "signature chaining"; it’s magic.

This is pretty incredible. With this signature chain you could be looking at a photo and see the entire chain of activity that has happened from the capture on the sensor to whatever you are seeing.

I've been saying for a while that we need to start signing everything. And for what it’s worth, the most advanced, easy-to-use, and readily available signing system out there is Ethereum.

Quoting myself from April 2023:

As AI makes the Web more synthetic, Crypto will make it more human.

I think this signature chain for photos is phenomenal. Where Ethereum could do it even better is by knowing more about those intermediate signatures, and connecting it to Ethereum Name Service so I know what organizations are involved.

All very cool.