The next time you are at a coffee shop and tap your credit card, you might marvel at how the transaction is approved so seamlessly. The interaction might seem simple, but behind the scenes, it involves a fascinating blend of physics, engineering, communication protocols, and cryptographic algorithms.

Based on the deep dive by Branch Education, here is a breakdown of what happens when you tap your card.

How the Card Gets Power Without a Battery Contactless credit cards do not have built-in batteries, meaning they must harvest power wirelessly from the payment terminal.

• The Terminal’s Magnetic Field: The payment terminal has a built-in NFC (Near Field Communication) antenna coil that generates an alternating current at 13.56 MHz. This creates an oscillating magnetic field extending a few centimeters above the machine.
• The Card’s Inductor Coil: Inside your credit card, there are intricate flat sheets of die-cut metal forming an antenna loop.
• Capturing the Energy: When these loops enter the terminal’s magnetic field, the field induces an alternating voltage. When paired with rectangular metal patches acting as a capacitor, it creates a tuned LC circuit that maximizes power transfer at exactly 13.56 MHz.
• Powering Up the Brain: This induced voltage is routed into the card’s tiny EMV (Europay, Mastercard, and Visa) chip, converting the AC voltage into a steady 1.8-volt DC to power the digital circuitry.

A Seamless Conversation: Sending Data Back and Forth Once the card is powered, the terminal and the card exchange a flurry of messages using specialized transmission methods.

• Terminal to Card (Amplitude Modulation): Since the terminal is already emitting an oscillating magnetic field to provide power, it uses that same field to transmit data. By creating short microsecond “pauses” or blips in the field’s signal—a technique called modified Miller encoding—the terminal sends binary data without starving the card of power.
• Card to Terminal (Load Modulation): The card has no transmitter, so it communicates by cleverly manipulating the terminal’s existing magnetic field. By rapidly turning a transistor on and off to connect and disconnect a resistor, the card changes how much energy it draws from the field. The listening terminal detects these high and low loads, interpreting them as binary ones and zeros.

Keeping Your Money Safe with Cryptography Before any money leaves your account, the card and terminal utilize highly advanced mathematics to protect against fraud.

• Proving Authenticity (Asymmetric Encryption): To prove it isn’t a cloned card, your credit card uses a unique secret “private key.” The terminal sends a random number, the card encrypts it with its private key, and the terminal uses a corresponding “public key” (digitally signed by Visa or Mastercard) to decrypt and verify the message. This ensures your private key never leaves the chip.
• Securing Transaction Details (Symmetric Encryption): To prevent hackers from altering the purchase amount in transit, the card encrypts the transaction details using a symmetric key shared only with your bank. This generates an Authorization Request Cryptogram (ARQC). The bank verifies this ARQC to ensure the details match perfectly before approving the purchase.

A PlayStation in Your Pocket Completing these intense cryptographic calculations requires a surprisingly robust processor. The tiny EMV microchip glued inside your credit card is a complex integrated circuit that is computationally on par with an entire original PlayStation 1 video game console.

So, the next time your purchase is “Approved,” you can appreciate the 72-plus steps of engineering, physics, and cryptography that took place to make it happen!