The "natural key" for a (natural) person is compound: full name and mother's full name, plus date, time and place of birth. Your birth certificate is your primary identification document.

However that still runs into problems of nondurability of the key in cultures that delay naming for a few years. To name one problem.

So yeah, use a big enough integer as your key, and have appropriate checks on groups of attributes like this.

However, if you are only interested in citizens, then a "natural" key is the citizen id issued by the government department responsible for doing that. (Citizen is a role played by a natural person so probably doesn't have too many attributes.) I still wouldn't use that as a primary key on the citizen table, though.