If you provide your log files by uploading them to Oncrawl's FTP server, you will need to use an FTPS connection.
When you connect to a website and need a secure connection, you make sure that connection is established in HTTPS and that it is secured with a valid certificate (represented by a lock).
FTPS is the equivalent for FTP, and it is as secure as HTTPS. It means 2 things:
the connection is encrypted in a secure channel, with SSL or TLS.
the server has a valid certificate and its identity can be verified
How to enable FTPS with my FTP client?
Some FTP clients, like FileZilla, will try to establish a secure connection by default:
If your FTP client does not use TLS by default or you do not want to send your password in plaintext, then you want to turn the option "Explicit FTP over TLS".
This option will force the client to establish a secure connection, and the connection will fail if it cannot be secured.