Retrieve encrypted sensitive card details, such as cvv and card_number.

Security Requirements:

1. Generate RSA key pair (2048-bit minimum).

2. Base64-encode the PEM-formatted public key, including the
   -----BEGIN PUBLIC KEY----- and -----END PUBLIC KEY-----
   markers and the newlines between them.

3. Send encoded public key in X-Client-Public-Key header.

4. Server returns cvv and card_number encrypted with your
   public key.

5. Decrypt response using your private key.