Enter the maze


by Jane Waite and Paul Curzon, Queen Mary University of London

A credit card in a back pocket. From PIXABAY.com

Contactless payments seem magical. But don't get caught out by someone magically scanning your card without you knowing. Almost \pounds 7 million was stolen by contactless card fraud in 2016 alone...

Victorian Hi-Tech

Contactless cards talk to the scanner by electromagnetic induction, discovered by Michael Faraday back in 1831. Changes in the current in a coil of wire, which for a contactless card is just an antenna in the form of a loop, creates a changing magnetic field. If a loop antenna on another device is placed inside that magnetic field, then a voltage is created in its circuit. As the current in the first circuit changes, that in the other circuit copies it, and information is passed from one to the other. This works up to about 10cm away.

Picking pockets at a distance

Contactless cards don't require authentication like a PIN, to prove who is using them, for small amounts. Anyone with the card and a reader can charge small amounts to it. Worse, if someone gets a reader within 10cm of the bag holding your card, they could even take money from it without your knowledge. That might seem unlikely but then traditional pickpockets are easily capable of taking your wallet without you noticing, so just getting close isn't hard by comparison! For that kind of fraud the crook has to have a legitimate reader to charge money. Even without doing that they can read the number and expiry date from the card and use them to make online purchases though.

A man in the middle

Security researchers have also shown that 'relay' attacks are possible, where a fake device passes messages between the shop and a card that is somewhere else. An attacker places a relay device near to someone's actual card. It communicates with a fake card an accomplice is using in the shop. The shop's reader queries the fake card which talks to its paired device. The paired device talks to the real card as though it were the one in the shop. It passes the answers from the real card back to the fake card which relays it on to the shop. Real reader and card get exactly the messages they would if the card was in the shop, just via the fake devices in between. Both shop and card think they are talking to each other even though they are a long way apart, and the owner of the real card knows nothing about it.

Block the field

How do you guard against contactless attacks? Never hand over your card, always ask for a receipt and check your statements. You can also keep your card in a blocking sleeve: a metal case that protects the card from electromagnetic fields (even using a homemade sleeve from tin foil should work). Then at least you force the pickpockets back to the Victorian, Artful Dodger style, method of actually stealing your wallet.

Of course Faraday was a Victorian, so a contactless attack is actually a Victorian way of stealing too!