September 20, 2020

Are DVDs Copy-Protected?

Maximillian Dornseif at disLEXia wonders why people refer to CSS, the encryption scheme used on DVDs, as “copy protection.” He points out, correctly, that encryption by itself cannot prevent copying, since encrypted bits can be copied just as easily as unencrypted ones. He wonders, then, how CSS can be called an anti-piracy measure. The answer is a bit subtle.

Dornseif is correct in saying that CSS by itself does not prevent copying. The goal of CSS is not to control copying (at least not directly), but to control who can build DVD players. In order to build a DVD player, you need to know how CSS works. If CSS is a trade secret, and if the owner of that trade secret will license it only to parties who agree to abide by certain rules, then all DVD players will abide by those rules. And if those rules make DVDs harder to copy, then the whole licensing scheme acts as a kind of indirect copy control scheme. Indeed, the owner of the trade secret can use this kind of scheme to enforce almost any limitation on use of DVDs, regardless of whether that limitation has any relation to copyright infringement.

The whole scheme breaks down, though, if the details of CSS get out to the general public. If that happens, then anyone can build a DVD player without having to agree to anybody’s rules (other than the legal rules that apply to everybody). Given the way players work, it was inevitable that somebody would reverse-engineer a player and recover the CSS algorithm. Learning a “trade secret” by reverse engineering is legal in many jurisdictions, so the CSS licensing scheme was doomed from the beginning to fail because somebody would legally reverse engineer CSS and publish it.

If you’re the DVD cartel, you want to do something to salvage your trade secret scheme. There are two ways you can do that. The first method is to make reverse engineering illegal (or just claim that it’s illegal), and then sue anybody who reverse engineers a player (e.g., Johansen), or anybody who publishes the results of reverse engineering (e.g. Pavlovich, Bunner, et al.). Alternatively, you can get a law passed that outlaws players that are made without your permission, and then sue anybody who make non-authorized DVD player or extractor products (e.g. Corley).

Both rules – banning reverse engineering, and banning unauthorized interoperation – are very dangerous from a public policy standpoint, as many of us have argued. But that hasn’t stopped vocal advocates from endorsing them, and even getting limited versions passed.


  1. DVD-CCA claims CSS is Copy Protection

    But CSS can’t be a copy protection. We have been through that. CSS purpose is enforcement of the terms of a cartel/monopoly. […] So a kind reminder: Beware of Fuzzy Language!