Using a Computer Safely
The trail you leave online
Work out how few harmless-looking facts it takes to narrow a crowd down to one person, and tell what a website is handed by you from what it is handed anyway.
Every time you use a service you hand it something. Some of it you type in on purpose — a name, a school, a date of birth. Some of it you hand over merely by turning up: a website is told the address your computer is using, which browser you are using and what time you arrived, whether you type anything or not. On its own, no single piece of that looks worth worrying about, and on its own no single piece usually is. The question worth asking is a different one, and it is a counting question.
A fact does not have to name you to find you
Each fact somebody knows about you cuts the crowd down to the people that fact is true of. Cut enough times and one person is left — and from that moment every other fact on the list belongs to that person by name. So a fact picks out one person exactly when no two people in the group could have the same one, and a set of facts picks out one person exactly when no two people in the group could have the whole set. The dangerous step is rarely the handing over. It is somebody laying two lists side by side that were never meant to meet.
Worked example
There are 24000 people at a concert. Half of them came by bus; one in ten of those is in the same year at school as you; one in four of those plays a musical instrument. How many people fit all three?
Half of 24000 came by bus, so 12000 are left.
The crowd is now everybody the first fact is true of, and the 24000 is finished with.
Try it together
One website stores a nickname and a birth date for each of its 50 players. A second website stores a birth date and a real name for the same 50 people. Neither has been told anything the other holds.
The two lists share one column, and that is enough to work with.
1.Suppose no two of the 50 players share a birth date. How many of the 50 nicknames can be turned into a real name by laying the two lists side by side?
Have a go
Have a go on your own. A group has 480 people in it. One in six of them wears glasses, and one in four of those is left-handed. How many people fit both facts?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.