Hol up. So what stops you from uploading custom photo + metadata onto random chip and planting it in a fake passport?
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So you could implement a chip which reacts like an official passport. When the border guards see that the signature is invalid, you can explain how it's just a prank and you'll all have a jolly good laugh about it.
I'd consider that pretty unlikely. Degraded chips would most likely provide no signature, not an invalid one. (Being able to randomly flip bits would be a big security problem for these kinds of ICs, so I'd assume they'd have robust protections against that.)