You can use an SSH key to use to authenticate SFTP connections. Upload each key that you want to use in the Data Management section of Marketing Cloud Engagement Setup.
Key Requirements
Use a tool such as ssh-keygen or PuTTYgen to generate an SSH key. The SSH key you generate must follow these standards:
- OpenSSH2 key in PEM standard
- At least 2048-bit strength, with a recommended strength of 4096 bits
- RSA format
- A passphrase is allowed but not required
Marketing Cloud Engagement supports OpenSSH 9.1.