Money and cards Episode 1 Sources: 7

How does a contactless card pay in half a second?

The short answer

The card has no battery: the reader's radio field powers the chip for a split second, and the chip creates a one-time code for that single payment, so a copied code is useless.

Bank card: shine the torch to look inside

The full explanation

Your bank card has no battery. So how does it pay in half a second? When you tap, the reader sends out an invisible magnetic field.

That field powers a tiny chip and an antenna coil hidden inside your card. It creates a secret one-time code for that payment, and that code never works twice. Mastercard even requires the card to do its part in under a third of a second.

Sources

Insidely's promise: every fact in this episode comes from these sources.

  1. No battery, powered by the reader's field (NFC, 13.56 MHz, ISO/IEC 14443): research paper arXiv 2302.08992, https://arxiv.org/abs/2302.08992
  2. A one-time code (cryptogram) for each payment that can't be reused: US Treasury Bureau of the Fiscal Service, EMV Merchant 101, https://fiscal.treasury.gov/files/cas/EMV_Merchant_101.pdf
  3. The same, in plain English: Spark glossary, EMV chip, https://www.spark.money/glossary/emv-chip
  4. Mastercard requires card processing under 300 ms for contactless products: Infineon press release, 16 Nov 2015, as published by the Secure Technology Alliance, https://www.securetechalliance.org/new-payment-platform-for-secured-and-ultra-fast-contactless-transactions
  5. Under half a second of interaction between card and reader: Visa patent US 7,798,394, https://patents.justia.com/patent/7798394 (also EP 2711889, https://data.epo.org/publication-server/rest/v1.2/publication-dates/20140326/patents/EP2711889NWA2/document.html)
  6. Antenna coil inside the card, outermost turn close to the card edge (full-size coils): NXP card coil design guide AN12342, https://www.nxp.com.cn/docs/en/application-note/AN12342.pdf
  7. Coil designs vary, and smaller antennas are used especially on embossed bank cards: research paper arXiv 1507.06427, https://arxiv.org/abs/1507.06427 (see also TU Graz, Optimizing Design of Smaller Antennas for Proximity Transponders, NFC 2010, https://www.computer.org/csdl/proceedings-article/nfc/2010/3998a077/12OmNylsZUz)

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