Better yet, stop storing the damn thing AT ALL. Personally, the idea that an SSN is some super-sekret uber-key to steal somebody's identity is just STUPID, in and of itself. I get that it's got historical inertia, but when we depend on a *publicly used* ID to establish one's identity, along with other *publicly used* information, well, words fail me.
We've never correctly solved the authentication problem and may never, but until we do, requiring SSN for anything than its original purpose is just criminal.
Besides, isn't there a federal law prohibiting the use of SSN as an identification number?