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.
Message Level Encryption (MLE) is required for all Card-On-File Data Inquiry implementations. MLE provides an enhanced security for message payload by using asymmetric encryption technique (public-key cryptography). The Card-On-File Data Inquiry request body is expected to be encrypted and the response body from the API will be encrypted as per MLE. You can generate the encryption/decryption key pairs in the Sandbox, Certification, or Production environments. For details, refer to the Message Level Encryption Documentation.