ISO 8583 Specifications: Hands-on experience with Visa Base I, Mastercard MIP/Debit, Pulse, or Fiserv STAR/Accel specifications (messages 0100/0110, 0200/0210, 0400/0420, 0800 network management, DE55 EMV TLV data). Card Payments Domain: Understanding of EMV chip/contactless flows, stand-in processing, settlement, chargebacks, HSM integration (Atalla, Thales payShield), and key management (KEK, ZPK, key blocks).