As one of the security protocols, Visa Developer sandbox secures its connections with clients by means of Two-Way SSL (Mutual Authentication) method. Refer to the link below to quickly learn about how to get credentials to start building with Two-Way SSL.
Visit the Two-way SSL Guide to learn more.
Visa Developer also provides the “Hello World” sample code that aims to demonstrate the connectivity with the Visa Network using the Hello World API and using the authentication method - Two-Way SSL (Mutual Authentication).
Once you sign in with your Visa Developer credentials, you can download these sample codes from the Project Dashboard > Assets.
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API Overview
API Reference
Working with Visa APIs