IBM to Provide Security w/o Sacrificing Privacy Using Hash Functions

Alex Mauer hawke at
Tue May 24 22:34:05 CEST 2005

Sean C. wrote:

> I'm confused though.
> I just read this article from the New York Times. As a newbie to encryption and
> hash algorithms I thought the idea behind hashes was that you couldn't
> reconstruct the data from the hash.

You can't.  But you can use the hash as a key to cross-reference

For example, if they were using full names to generate the hash:

Watch list contains:

"Alex Mauer" -> foo
"Billy Z Williamson" -> bar
"Corgi McCorkerton" -> baz

Passenger list contains:
"Billy Z. Williamson" -> xyzzy
"Alex Mauer" -> foo
"Fenster LeCrab" -> baz

...they only need to compare the hashes, for the cruise ship company to
see that "Alex Mauer" should not be allowed to board, and the government
to see that "Alex Mauer" attempted to board a cruise ship.

The government doesn't need to reveal their watch list to the cruise
ship company, and vice versa.

This also illustrates some problems with the system, namely hash
collisions (two people generate the hash "baz") and the fact that slight
changes in data will lead to totally different hashes (added period
after middle initial).

-Alex Mauer "hawke"
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