Social Security Numbers are the most common type of TIN. The SSN is a nine-digit number in a XXX-XX-XXXX format. It’s used by American citizens, permanent residents, and temporary noncitizen residents with authorization to work in the US. You must obtain a SSN to work within the US. SSNs are issued by the Social Security Administration (SSA), not by the Internal Revenue Service. To get a new SSN, you must apply in-person at a SSA field office.