Yeah, social security number.

It's basically a private key we're all assigned at birth that can't be rotated. It was originally meant to be used in the state-sponsored welfare program "social security", but tons of businesses require it as a unique identifier.

Absolute stupidity on all fronts.

I'm pretty certain you can ask them for a random number instead, as only the government is allowed (last I recall) to __require__ an SSN. I know my doctors can ask for it, but are required to accept an alternative if I don't wish to provide the SSN.

For example:

http://www.identityhawk.com/Who-Can-Lawfully-Request-My-Soci...

Sadly, it's too late in the day for me to try to parse this: http://www.justice.gov/opcl/social-security-number-usage