But it requires the attacker to know a lot of information in the first place: 4 time/date pairs. In other words, if you already know a lot of information about

But it requires the attacker to know a lot of information in the first place: 4 time/date pairs. In other words, if you already know a lot of information about a person, you can get even more information from the "anonymous" data set. Why is that surprising?