SSN encryption

drfremove_at_nber.org
Date: 09/29/05 

Date: 29 Sep 2005 05:25:17 -0700

We want to encrypt social security numbers in a database. The special
requirement is that the same SSN should encrypt to the same value
regardless of context. That way when new records are added to the
database it will be possible to easily match them to existing records.
Also, the tables can be sorted or selected and still matched to other
tables in the database.

This has been discussed in this newgroup before, but as far as I can
tell, the solutions are of the form "Map each SSN to a sequence number,
and compare to the list of already seen SSNs to know when to assign a
new one." However this means keeping the cross ref online for each new
file, which almost defeats the purpose of encrypting the SSN (at least
in our situation).

Since the intruder can probably identify his own record (and those of
close relations) from the non-SSN content, the method needs to have
some degree of security from known plaintext attacks. There are weak
clues to the first 5 digits in the data content (such as region codes),
if that is a consideration. Although the raw files are sorted by SSN,
we would resort tables by the encrypted SSN, to destroy that bit of
information. (An algorithm that maintained sort would be great, but we
assume insecure).

Daniel Feenberg
feenberg isat nber dotte org

Relevant Pages

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  • Re: SSN encryption
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