Fusion expense - Accessing Credit Card file over SFTP using key based authentication

Summary:

In Fusion Expense, We need to access credit card file placed on SFTP but connection to SFTP should be key based.

Content (required):

We have requirement to process mastercard file. This file will be placed on customer's SFTP server by the bank. But due to audit requirements, we can't access the SFTP using user and password. SFTP connection should be established using keys only. In fusion expense setup, it requires user/password and there is no place to provide keys for connection establishment. 

Has anyone come across similar requirement and how it was achieved.

Version (include the version you are using, if applicable):