SFTP protocol runs over SSH channel and the SFTP protocol is secured by default (unlike the FTP protocol, which is still often used over unsecured TCP). Please check our differences between FTP, FTP/SSL and SFTP protocol article to learn more.
This means that even the following simple code is sufficient:
// create SFTP client instance
var sftp = new Rebex.Net.Sftp();
// connect to a server
// check sftp.Fingerprint here