Not sure how much I am able to say, it's already several years old but the bottom line is there are so many different documents that it's not always possible to

Not sure how much I am able to say, it's already several years old but the bottom line is there are so many different documents that it's not always possible to check the digital signature, either because it does not have a digital signature (we are not only talking about passports but all possible documents), can't read it because the chip is damaged, or has no way to verify the signature because the scanner does not have the certificate chain. And there are attacks where you simply forward the communication over the network to a remote reader so that the gate thinks it's talking to a real passport but the physical passport is somewhere else. And older passports use weaker hash functions to verify the integrity of the data