Social Security Cards

In the US, Social Security cards are issued by the Social Security Administration. The card, or rather the number displayed on the card, is the de facto national identifier for US citizens, especially for tax and credit purposes. The original card is best kept carefully safeguarded and only presented as proof of citizenship and the right to work in the USA. The 9-digit number displayed on the card is privileged information which, if it becomes public, could be used to steal one's identity.