In Connectivity studio, for a file action of type SFTP,  you can use a private key to access the SFTP server. You upload the private key certificate for the SFTP server from a file, usually of type PKK.
If you cannot connect to the SFTP server, using the private key certificate:

1. Check the private key that is used.
2. If the private key is OK, check the private key certificate file. Change the PKK file version to, for example, '2'.

To change the private key certificate file, you can use, for example, the PuTTYgen key generator tool. With this tool:

1. Load the private key certificate file.
2. Make the desired changes.
3. Save the private key.
4. Upload the saved private key certificate file to the SFTP file action in Connectivity studio.
5. Test the SFTP file action connection.