As Dat says, as long as you have an SSL certificate configured and send the request over HTTPS then you will be fine. This is not just a Kentico issue, the same applies whenever you are sending or receiving sensitive data across the web.
In fact, many browsers these days will display a warning to the user if they are accessing a site over HTTP instead of HTTPS, so you would be best to enable HTTPS across the whole site and completely disable HTTP, unless you have a very good reason for keeping HTTP enabled.