John Higdon wrote:

It is rather difficult to "make up" and SSN. Like credit card numbers,
they have checksums. A bogus SSN will drop a flag the moment it enters
an SSN-savy computer.

It's not a checksum at all, but just a series of rules which are available to
anyone who knows anybody who has done it. Even Wikipedia has posted the rules
for the SSN:
http://en.wikipedia.org/wiki/Social_Security_number