Building a Payment Applet on Java Card: From JCOP4 to EMV Contactless

Java Card is the platform that runs inside billions of smart cards worldwide. If you've ever tapped a Visa or Mastercard, you've interacted with a Java Card applet. We built our own. Here's what that looks like. What Is Java Card? Java Card is a stripped-down Java environment designed for smart cards and secure elements. It runs on chips with: 1-5 KB RAM 32-128 KB EEPROM 8-32 bit processors Despite these constraints, it supports cryptographic operations, secure storage, and the APDU (Application Protocol Data Unit) communication protocol that EMV relies on. JCOP4: Our Chip Platform JCOP4 (Java Card Open Platform 4) is NXP's latest Java Card platform. It supports: Java Card 3.0.5 API GlobalPlatform 2.3 P-256 ECDSA (critical for our use case) ISO 14443 contactless interface Common Criteria EAL5+ certification The OpenPasskey Applet Our applet does four things: 1. Key Generation On first initialization, the applet generates a P-256 key pair inside the card's secure element. The private ke