[liberationtech] need advice on using hashes for preserving PII's utility for disambiguation while protecting sensitive info

[Tomer Altman]

I'm not sure if this exactly addresses your needs, but I believe it is relevant:

https://en.wikipedia.org/wiki/Differential_privacy

HTH,

~T

----- Original Message -----
From: "Tom Lee" <tlee at sunlightfoundation.com>
To: liberationtech at lists.stanford.edu, sunlightlabs at googlegroups.com
Sent: Thursday, February 6, 2014 12:49:14 PM
Subject: [liberationtech] need advice on using hashes for preserving PII's utility for disambiguation while protecting sensitive info

We've been kicking around an idea at Sunlight that aims to use cryptographic ideas to resolve some of the concerns around the publication of publicly identifiable information in government disclosures. I could use some smart people to tell me what's dumb about it. 

We often face challenges related to disambiguating entities: is the John Smith who gave political donation A the same John Smith that gave political donation B? One obvious solution to this problem is to push to expand the information that's collected and disclosed -- if we had John's driver's license number (DLN), for instance, it'd be easy to disambiguate these records. But that could introduce privacy concerns for John. One approach to this problem (which I don't think government has tried) is employing a one-way hash. 

Obviously the input key space for DLNs and most other personal ID numbers is so small that reversing this with a dictionary attack would be trivial. You can add a salt, but only on a per-entity basis (not a per-record basis) if you want to preserve the capacity to disambiguate. That in turns calls for a lookup table in which the input keys are stored, which kind of defeats the point of using a hash (you might as well just assign random output IDs for each input ID). I would worry about government's ability to keep this lookup table secure, and I worry about the brittleness of such a system. 

Alternately, you can use a single system-wide secret (or set of secrets) to transform inputs into reliable outputs. I think this is less brittle and maybe easier to preserve as a secret, but this system might be too easily reversible given the ability to observe its outputs and know the universe of possible inputs. I'm unsure of the cryptographic options that might be appropriate here. 

For all I know, the lack of implementations using this kind of one-way transformation isn't about government sluggishness but rather about its feasibility. I'd be very curious to hear folks ideas on this score, though. My general hunch is that something must be possible -- even a few bits' worth of disambiguating information would be hugely useful to us, and presumably you're not leaking important amounts of information by, say, sharing the last digit of a DLN. So there must be a spectrum of options. But as is probably apparent, I don't think I've got a handle on how to think about this problem rigorously. 

Tom 

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