In the United States, a Selective Service Number must be applied for by all male citizens turning age 18. An optional national identity number is the Social Security number (SSN), a nine - digit number issued to U.S. citizens, permanent residents, and temporary (working) residents. Its purpose was to identify individuals for the purposes of Social Security, but it is now also used to track individuals for taxation purposes. There is no legal requirement to have a SSN if it is not required for Social Security or taxation purposes, but in practice one is required for many other purposes, for example to open a bank account or apply for a driving license, so that nearly all U.S. citizens and permanent residents have one. The SSN has therefore become a de facto national identification number, despite the fact that originally it was expressly not for this purpose. In fact, a valid SSN can be easily guessed, as they were issued serially prior to June 25, 2011.
how many digits is a social security number
nine