How do the apps that scan the NFC chip in a passport to ZK proof prevent fraud?

What type of fraud are you curious about?

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Allegedly a designer.

Input > ZK proof that I'm from the US based on a passport

Just fake the input?

What do you mean fake the input, though? You need to hold a passport. Are you asking "what if somebody else gets access to my private key"?

No, I'm saying what if proving you live in country X is valuable.

Wouldn't you have people creating fake NFC chips with seemingly real passport data from country X?

How do they know that the key is secure and hasn't been leaked?

The proof comes from you signing a message with a key that is probably issued from the country.

Basically, all countries have a known public key and act as certificate authorities, kind of.

So, because math, I don’t think you can fake having one of those keys.