“But how does anyone know what his SSN is?”
It’s on his tax returns and other transactional documents.
He sealed his college records and such, but he can’t ‘seal’ every piece of paper that shows his info for the past 50 years.
I have never seen this article but it is quite interesting:
http://www.birtherreport.com/2013/09/p-exclusive-obama-social-security.html
P&E Exclusive: Obama Social Security Number; Selective Service Card Signed By Current Admin