Comment by bobthepanda

Are we really going to go to a fictional TV show now?

Sure you aren't watching Psych?

What about a system that saves in some way the hash in a Blockchain, and if you, eg, XOR the hash of the video with the hash of the previous block you will "certainly" know that the video was created between the previous block and the block where the hash is saved in. That's a starting point.

"Crack the hash"? Does this mean you were employing some novel hashing algorithm and relying on its secrecy? If so your employer were never serious about security in the first place. Hardware attestation is more or less a solved problem, and that solution does not involve secret algorithms.

Now sell them version 2.

> But I don't know of a cryptographic mechanism to ensure that a digital image is not more recent than a particular time

Many (most?) blockchain mechanisms include a timestamp in each transaction on the chain, so while multiple records from the same owner prove little (the timestamps could be faked over a given period of time) the interaction with the wider network and the chain would give some confidence that the record happened between within a small amount of time.

The other possibility, that doesn't require a chain with many independent active participants, is to have things signed by an external trusted authority. Submit a hash of the content and appropriate metadata to them, and have them sign it with a signing timestamp. I've considered abusing ACME certificates for document signing like that: the hash of content (or some signature based upon it) becomes the subdomain to sign¹ and you get a certificate that even after expiry is evidence that the CA saw that value at the signing timestamp. Note of the signing will also be in the public certificate transparency log. This wouldn't, on its own, prove anything about the authenticity of the content, that could have been doctored before signing, but it does prove that the content+metadata existed at that time (so might be more useful in copyright claim type cases, or agreed contract situations where all parties have signed the content and the signatures are included in the metadata, than for proving authenticity).

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[1] based64²-ed with non-alphanumeric characters removed and truncated³ to fit or split, so acodha3sf7whsrhtqestkabtx0b4bbhyveee0ajnrpqcuxrjjvmhsujgcex.domain.tld or acodha3sf7whsrhtqestkabtx0b4bbhyveee0ajnrpqcuxrjjvmhsujgcex.w5jmmkpmyfgshx2jecsfordpnq.domain.tld

[2] names not being case-sensitive drops some of the entropy, if that is a concern use a 32-bits-per-character encoding instead and have names twice as long

Publish hash(image) on the blockchain at a verifiable time, then publish the image itself.

The image contains the previous block’s hash.

Wouldn’t this establish both a lower bound and an upper bound on the time the image could have been produced?

wouldn't that be a hash of the image signed by a trusted entity and stored on a chain? maybe i'm overlooking why this doesn't work

Interesting, There aren't any newspapers left in my country, neither printed nor not printed. The closest you can find is the weekly advertising booklet here and there. Which is irrelevant now because a computer can either stich new content to an old picture, or entirely producing a custom picture.

That would be a use case for a block chain. But I still don't understand how you are securing the integrity of the validity of the certificate stating the authenticity of the media. I only understand you are stamping media with a "at least as old as [timestamp]

Plenty of people. If you have running water, some tape, and trashbags, you too could have a darkroom.

https://www.metmuseum.org/exhibitions/objects/objects@exhibi...

The roll itself can be manipulated too. Most of the techniques used in modern photoshop are basically 1:1 carry overs of darkroom processes. Layers, dodge and burn, masking, etc.

There was a time you could take this class in highschool.

You can burn negatives. You can fake polaroids, really, just think about how a camera itself must operate and you'll see why instantly. Darkrooms used to be far more common before digital photography my Junior and High school both had them.

What makes evidence "pixel perfect?" What digital photographs don't have to involve a chain of custody? Literally the first question the defense will ask is "how did you get this picture." If you say you pulled from a security system they can just go ask for the originals. This happens all the time.

Where people are getting confused is it's almost never _one_ piece of evidence that's used to convict you; although, it may be a single piece of evidence which convinces your attourney to railroad you into a plea deal.

Not really. End result is the same: manipulated image.

It is really not any different. People would throw a hubcap in the air and pitch it as a UFO photo and idiots would latch on to that. You could take a photo of the empire state building and use a double exposure to make it look like you were king kong. Kids were doing this sort of stuff. Stop motion home movies where you'd look like you were levitating or your head got cut off.

It always comes down to provenance.

Plenty could.

https://www.metmuseum.org/exhibitions/objects/objects@exhibi...