source? cuz he's been opposed to nsa spying, trans pacific partnership (super long copyrights, patents, companies can sue countries for regulating, etc), etc etc for a while now. Seems pretty libertarian to me.
Libertarians don't want a bunch of government programs like Bernie does.. Libertarians want the federal government to have as little power as possible, and to have local governments take care of those kinds of issues or the free market in general. Something Sanders is very much against.
Just because he's against certain draconian laws doesn't mean he isn't an authoritarian.