NFC Tap-to-Pay Cryptogram Engine

A mobile wallet never puts your card number on the air. This simulator renders the phone-to-terminal NFC exchange in 3D and drives it with the real mechanic that protects a tap: a device-bound key and a strictly-increasing transaction counter (ATC) that together produce a fresh, single-use cryptogram on every tap. Trigger a payment to watch the counter advance and a new cryptogram travel across the NFC field, then launch a replay attack that resends a previously captured cryptogram unmodified — and watch the terminal reject it every time because its ATC has already been spent, exactly as EMV contactless / Apple Pay / Google Pay behave in the real world.