It's so wildly insecure that the holder identification number comes out in the mail. Then if you change your address, they send a notification letter to the old address just letting you know that it was changed. This means if you move house, and don't get a mail redirection, whoever lives at your old address will receive a letter containing your holder identification number.

Maybe the blockchain can fix that?

It's ok, because they often make you give your postcode as a second factor.

Or last name. Which is also printed on the letter…