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

Chris Dary umbrae at
Thu Feb 6 13:19:49 PST 2014

It's been a while since I dug into it, but something like an 8-bit
probably provide enough disambiguation but would collide often enough
to not be much of a concern for reversing - 256 different values.

On Thu, Feb 6, 2014 at 4:10 PM, Chris Dary <umbrae at> wrote:

> Just one thought to throw out: Something that sprang to mind is the idea
> of a check digit or simplified hash that would be redundant enough to
> collide very often if you were trying to reverse, but would still provide
> enough disambiguation that you'd be able to appropriately determine who
> you're dealing with.
> You could probably use something similar to the Luhn algorithm for that,
> although I'm not sure how uniform that is:
> - also, that only ends up
> with a single check digit, which is probably too small for good
> disambiguation. The approach in general might still be helpful though.
> -Chris
> On Thu, Feb 6, 2014 at 3:49 PM, Tom Lee <tlee at>wrote:
>> 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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