I need a secure, centralised payment gateway that plugs straight into my mobile app on both iOS and Android. The goal is simple: let users pay with any major credit or debit card inside the app without being pushed to an external browser, while keeping every transaction PCI-compliant and tokenised for maximum security. You will design, build, and document the full stack of the gateway: backend API, merchant dashboard, and lightweight SDKs (Swift and Kotlin) that my development team can drop into our existing code-base. The service must authorise, capture, refund, and void transactions, return clear error codes, and expose webhooks so we can update order status in real time. Deliverables • RESTful (or GraphQL) API with authentication, sandbox, and production endpoints • iOS and Android SDKs with sample checkout screens, card tokenisation, and 3-D Secure flow • Deployment scripts/instructions so I can host on AWS or any standard cloud environment • Brief but complete technical documentation and a postman collection for quick testing Acceptance criteria • A test card purchase from both iOS and Android confirms with a success callback in under 3 s • Failed payment returns the correct error code and message • End-to-end refund flow completes and logs in the dashboard • All sensitive data is encrypted at rest and in transit, passing PCI DSS checks If you have built card gateways or mobile payment SDKs before and can move fast while keeping security tight, let’s get started.