I have always used Filezilla and WinSCP for my SFTP transfers because they support SSH certificate authentication (actually using a key I generated in PuTTY - puttygen.exe). Actually, almost all FTP and telnet programs support SSH certificate authentication.
SSH certificate authentication allows you to connect to a server using a public key instead of a password (which is much more secure).
support.ssh.com/manuals/server- ... -cert.html
Does Directory Opus support this? I couldn't find it anywhere in the options. Maybe I'm just looking for the wrong thing or in the wrong place?