>I know that in Italy the tax number is used quite a lot. I could not buy a rechargable smart card for public transport, because I did not have the tax number.

>I know that in Italy the tax number is used quite a lot. I could not buy a rechargable smart card for public transport, because I did not have the tax number. And when just for fun at a train ticket machine I tapped that I would like a receipt, it asked me for my tax number. Whether the number is used as an authentication method and could be misused in that context I have no idea. No, the number is just a number, and as said it is not secret and can be generated (though with not a 100% guarantee of it being valid because of "total" homonimy). It is only an ID number, whilst the card is (can be) a form of online ID (but only on a few specific government sites) and the said fun fact as proof of age on vending machines. In any case it is never a password, so having it public it is not an issue, at the most you could have typed my (or someone else's) Codice Fiscale on the ticket machine, but you couldn't have used it for (if appliable) tax deduction on your name.