For a long time, the last 4 digits of a SSN were the only ones with any entropy. The first 5 were determined by when and where you were issued an SSN (usually

For a long time, the last 4 digits of a SSN were the only ones with any entropy. The first 5 were determined by when and where you were issued an SSN (usually at the hospital when you were born). Suppose your number is 123-45-6789. If an attacker can uncover that you were born in Springfield in December 1989, they can deduce that the first 5 digits are likely 123-45. Now, there are only 9999 possibilities for what your full SSN might be. Moreover, the only random bits are routinely used by utility companies, hospitals, etc. as an oral password and written identifier. If someone reads or overhears that your last 4 are 6789 and knows when/where you were born, they have a frighteningly good chance of discovering your SSN.