What's the point? If your network can't be trusted, that is, there might be someone masquerading, there's no point sending over the file unencrypted. If someone can masquerade, that someone can just snoop the network. Which means that your confidential data can be read anyway.

Samba allows for a username and password to do some authentication at mount time, but I do not know how that password is being exchanged.

I wouldn't do any exchange of data using samba or NFS if my network can't be trusted - in such a case, I'd use ssh or something similar.