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The problem with social security numbers today is that some organizations are using these ubiquitous numbers for identification, others are using them for authentication, and still others are using them for both. I call up my bank, tell them my account number and ask them for a balance. Just to make sure that I am really who I claim to be, my bank asks for my SSN—as if this is a number that is some kind of secret that we share.
Simson Garfinkel (Sun,) studied this question.