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But everybody can give his/her smartphone to a 3rd party to carry around, right? Your Google location history only proves where your phone was, it doen't prove where you were.

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That’s why you have to combine it with selfies and whatnot on Instagram and/or Twitter, preferably in front of identifiable landmarks or storefronts.

You mean you're using online services to provide you with trusted timestamp? [1]. But you need to consider how a trusted timestamp merely proves that some photo was definitely taking at or before the attested time. It does not prove that a photo was not actually taken a day earlier and then merely submitted at the attested time. To prove something was not recorded earlier is a little trickier. Need something like a newspaper headline that would not have been available earlier. And then how do you defend against claims that you took the photo early, but photoshopped the newspaper headline at the time it was attested?

[1] https://en.wikipedia.org/wiki/Trusted_timestamping

True, maybe Instagram can roll out a feature where it grabs the GPS data to verify location on the photo, or signals that the photo was not uploaded but rather taken.

Each item isn’t meant to be foolproof, just to assist in creating third party verifiable data when combined with things like credit card transaction data or road toll timestamps to make it improbably for anything else to have happened.

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