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London's tube demands faster-than-NFC ticketing

500 milliseconds not quick enough for the Underground

Application security programs and practises

London Underground has been telling NFC World that existing NFC implementations just aren't fast enough, so it will be sticking with plastic cards for a while.

An Oyster card tapped against the reader takes between 200 and 300ms to authorise, before the gates snap open as a prelude to grabbing your bag as you pass though, but trials of Near Field Communications technology have shown authorisation times approaching a second, which NFC World reports is not fast enough for those dashing to get a tube.

Exactly where the bottleneck lies isn't clear: NFC Times fingers the SIM's ability to complete the encryption process, which could be improved with dedicated hardware, but the Single Wire Protocol (which connects the SIM to the NFC hardware) could also be to blame, along with the complexity of Oyster tariffs. What is clear is that the process is going to have go get a lot faster if the Underground is going to start accepting credit cards embedded in telephones, as it would like to.

Near Field Communications is a standard comprised of three components: an induction-powered tag, a reader capable of inducing power in similar tags, and a secure element for storing and using cryptographic keys for authentication. For ticketing the reader component isn't used, so it's the tag and the secure element that matter.

Network operators would like the secure element stored on the SIM, connected to the handset over the Single Wire Protocol (SWP), while handset manufacturers would prefer the element embedded in the handset and banks have so far shown a preference for it to be located in a removable SD Card. The only two handsets currently supporting NFC – Nokia's C7 and Google/Samsung's Nexus S – both support SWP, but also have embedded secure elements under the control of the manufacturers (or Google, in the latter case, we still don't know for sure).

The SIM is certainly capable of fast encryption: it is required to decrypt session keys, and cryptographic challenges from the network, enormously quickly, but that's using hardware designed for specific cryptographic algorithms, which is much easier than generic cryptography. The Oyster network is moving towards using DESFire, an update to the cracked MiFare platform, so NFC implementations that want to do fast Oyster transactions will need to support that protocol in hardware to achieve the speed necessary. The embedded component in the Nexus S is supplied by NXP Semiconductors, owners of DESFire, so it should be able to cope, but a secure element dropped into a SIM might struggle, and that's before credit cards are taken into account.

Oyster is contracted to provide ticketing to the Underground, but Transport For London would much prefer to collect the money itself, which means taking credit-card payments at the turnstile. In theory, a PayPass or PayWave (Mastercard and Visa respectively) card could be tapped against the reader, authorising the reader to contact the card-holder's bank and deduct the cost of the journey from the holder's account, but doing it in less than than half a second is tough.

And it's 500ms that London Underground is specifying as the cap, despite having previously said that 350ms is the longest it is prepared to wait. The convenience of mobile-phone credit-card payments on the tube is apparently worth waiting 150ms for, but not a millisecond more. ®

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