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.
Some Visa Developer APIs require an API Key-Shared Secret Authentication, which Visa refers to as x-pay-token. To invoke an API using x-pay-token, you will need an API Key and a Shared Secret, which is provided via the link below.
Visit the X-pay Token Guide to learn more.
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Working with Visa APIs