Doesn&#x27;t 2-factor solve this problem?  How would registering for SSN + 2FA be any different than registering for a SSN?<p>Turn the SSN into a (not secret) username&#x2F;uid and add a 2-factor seed that&#x27;s a shared secret between the SSA and the individual.  Provide an API for 3rd parties to validate (SSN, 2FA code) pairs.  If things go wrong, 2FA seeds are lost, etc, there&#x27;s the same fallback that exists for lost SSN cards or stolen SSNs: go to a SSA office and get it fixed in person.<p>The SSA could provide an online SSL-protected client-side webapp to generate 2-factor codes for those who don&#x27;t have a smartphone.  They could offer it as a telephone service too.