...........thats not a promise you can make. Infact, I'd argue that you're inviting to get hacked, because now some hacker is out there like CHALLENGE ACCEPTED!
The easiest way is to not store the data in the first place. Can't hack data that doesn't exist. Why does anyone need to keep social security numbers on file.
Especially considering they were explicitly designed NOT for that purpose. Not only is it illegal for government agencies to deny you anything if you refuse to provide an SSN, but until 1972, this verbiage was printed on every Social Security card: "FOR SOCIAL SECURITY PURPOSES -- NOT FOR IDENTIFICATION."