weierstrass's Journal: the hacker property

my little sister surprised me; we were talking about 'oyster' cards, the rfid-based fare system for most public transport in london. the system is quite new and quite complex as there are lots of different fares and different ways of paying; you can put credit on the card and get charged per journey, or use it as a 'travelcard' which allows you unlimited travel through certain zones over a day, a week, a month etc., or use a mix of both.

my sister has used it for a while and when i talk to her about it she tells me the different ways she 'swipes' in and out of the system so as to avoid paying where it's not necessary (if the station has the barriers open / no barriers) or so.

i'm pretty interested in 'oyster' so i'd been doing a little research into it. i explained to her that there are two ways you can run a system like this. one is that the cards have no information on them except an identifier. when you 'swipe' this just allows the barrier or ticket machine to look you up on the central database, to check if you're allowed to travel, and modify the amount of credit you have if necessary. the other way is that this information is actually stored on the card in some form, which would presumably need to be encrypted. often when i talk to her (and many other people) about stuff like this i get the feeling they are just humouring me by acting interested.

then she surprised me by telling her what i was just about to tell her: "Yes, oyster seems to use a mixture of both." I knew this from reading information on the internet about it, but she didn't.

from observing the behaviour (input and output) of a black box, she was able to create a theory about how the system worked inside. that is the hacker property.

Absolutely

[Marxist Hacker 42]

Give that girl a gold star and a nice scholarship- she's going to need it. Ask her the one about the two black boxes that are electronically identical in output- how to tell which one is the current source. It's an interesting answer.