[charlesdaniels]

> Unless several topics can be assigned to a person (which seems to be implied in the article), in which case that's 256 bits of entropy available to classify each person.

Good catch, forgot this was a bit-vector not a single key.

> Yeah, well theoretically you could. But that assumes that browsers are able to extract and balance some very arbitrary and very specific information from the browsing habits of all people on earth in a perfect decision tree.

Not really, people have found in the past that combinations of user agent, screen resolution, installed fonts, installed extensions, and things of that sort can come very close to uniquely identifying individual people.

> Though I think you are absolutely correct that in practice the number of bits to build up a classifier able to uniquely classify each person must be pretty low. Maybe a few hundreds.

Exactly. It might not narrow it down to one person, but perhaps a relatively small pool.

[jnwatson]

I imagine that advertisers wouldn't have access to the entire bit vector.

Google would also have limit the number of bits an advertiser has access to.