[Cryptography] De-Anonymizing

[John Gilmore]

> I generally pay with cash at the local supermarket.  But I have one of 
> those store tracking things that a mate gave to me.  So his purchasing 
> pattern is intermingled with mine.  Which is probably not a lot of 
> confusion, can be stripped out if it is known.
> 
> Perhaps we could do a mix?  If we each meet once a month, and we throw 
> our supermarket tracking things into a basket, mix it up, then everyone 
> takes a random one.  Next month, remix.

The classic way to do this is to swap cards with other people who
you happen to meet while waiting in line at the cash register.  Not
only do you point out to them how any card will do, and how the
retailer never told them that the whole point of the cards was to
track them, but you will 'mix' with people you never saw before and
are likely to never see again.

Also, at some of those stores, you can get a new card every time you
go, and it works even before you are "supposed to" fill out a mess of
personal details about yourself.  Tell 'em you never had one.
Generally the clerks are sympathetic rather than mentally bought into
the management's efforts to track everybody.

	John