> This is yet another alarming signal that the whole idea that your SSN/SIN or credit card number is somehow secret and can be used for authentication is flawed.
Secret information is required for practical authentication. Some examples:
I believe you don't often write down your private keys, passwords, api secrets or auth tokens on paper forms that you then submit to various institutions you have no control over, right?
As a comparison, your SSN is used all over the place, you will need to disclose it regularly (rent an apartment and your landlord is likely to request a credit check, you'll give him your SSN). A number of people and institutions will have access to it. It is not a secret. It should not be used for authentication and no one should assume it somehow secret, because it is not.
Secret information is required for practical authentication. Some examples:
Private keys (ssh, gpg, SSL/TLS certs, bitcoin wallets)
passwords
session ids
api secrets
auth tokens