[not2b]

I could take a photo of someone else's photo with a camera that cryptographically signs the image. Then I suppose I could claim that my photo is the original (see? it is signed, with a camera that maintains a chain of trust) and the original photo is now the stolen one. To pull this off it would have to be a really high quality camera that would make an accurate copy.

Perhaps something like this is what your hostile responder was thinking of.

[tornato7]

But if both images had a timestamp in the signature, wouldn’t you be able to prove that the original was taken first?

[AnthonyMouse]

How does the device know what time it is?

What happens if the original was taken with any existing device that doesn't make signatures, or is a model that subsequently had its keys revoked?

[tornato7]

You can store the hash on a public Blockchain and the timestamp of that transaction will be verifiable.

[AnthonyMouse]

If everybody does that with everything, blockchains get infeasibly large. If not, anybody can go and register things on a blockchain that the original creator didn't and then claim they were first.

[tornato7]

You would just use any sort of aggregation scheme to include multiple hashes at once. Even concatenating 1000 image hashes and hashing that would allow you to prove later that they were all included.

[AnthonyMouse]

That just moves the problem from where to store the blockchain to where to store the concatenated hashes.

If this is some cloud provider, what are you getting from a blockchain? Just have the cloud provider do the certification. If they betray you or go out of business you've lost your hashes anyway.

If it's stored on the endpoint device, you can't prove it anymore if the device gets lost or damaged. In theory people could back them up, but we all know perfectly well that ordinary people are not going to do that unless it's automated.

So then you're back to storing them in a distributed system, i.e. making them a necessary part of the blockchain. And then it gets too big.