Data transmission in NoMachine sessions is always encrypted using end-to-end encryption. This prevents third parties from accessing data transferred from one endpoint to another. This basically means that all session data (not just files being transferred, but also keyboard and mouse input as well as screen output) between a NoMachine client and a NoMachine server cannot be intercepted and decrypted other than by the client and server in question. Therefore, no third party, including your own Internet Service Provider, not even NoMachine' administrators by request from an authority can intercept this data, as only the sender and the intended recipient have the key to decrypt the information. 

For in-depth details about end-to-end encryption in NoMachine and what data NoMachine has access to, please consult the following:

NoMachine Security Statement
https://kb.nomachine.com/AR04S01121

NoMachine Privacy Policy
https://kb.nomachine.com/AR05P00977