Newstex supports requiring a username and password for any feed provided to us over standard HTTP. We support both Digest and Basic HTTP Authentication schemes. We also support HTTPS so you may ensure that your data is transmitted securely over SSL.
For more information on securing feeds by Apache, visit http://httpd.apache.org/docs/1.3/howto/auth.html
For more information on securing feeds using NGinx, visit http://wiki.nginx.org/HttpAuthBasicModule
If you are using WordPress, consider disabling your RSS feed entirely and using the Newstex Wordpress Plugin