But it can be done with zero knowledge proofs. Once Mike Hearn explained to me how that could work. If I recall correctly you could have a digitized passport instead of the analogues one we are used to. Some countries are already putting chips into their passports to make them digital documents in addition to their physical issues. Zero knowledge proofs enable an application to read the whole document and only check for specific information that you want to proof without giving away any other information that the source also contains. After parsing the doc it puts out a verifiable proof that the input document actually contains the requested information, eg that you are Chinese citizen or that you have a certain age or name. All the proof states is what you want it to verify without giving away further details.