What exactly is DRM circumvention, as opposed to legitimate use?

Question

DRM only makes media useless if it cannot be decrypted. What makes it legal for my commercially made DVD player to decrypt a disc, but not for libdvdcss to do it? DVDs don't have labels saying that use of libdvdcss is not allowed. If both are being used only to watch the video in the appropriate region, is there any meaningful legal difference between the two CSS decryptors? Can a copyright holder say "this disc may only be decrypted with libdvdcss; use of any other software is unlawful circumvention"?

Answer 1

This is explained in the law.

(1) (A) No person shall circumvent a technological measure that effectively controls access to a work protected under this title.

The law also defines two expressions:

(A) to “circumvent a technological measure” means to descramble a scrambled work, to decrypt an encrypted work, or otherwise to avoid, bypass, remove, deactivate, or impair a technological measure, without the authority of the copyright owner

and

(B) a technological measure “effectively controls access to a work” if the measure, in the ordinary course of its operation, requires the application of information, or a process or a treatment, with the authority of the copyright owner, to gain access to the work.

Wording in a license is not a technological measure. It means that there has to be something in the hardware or software that enforces (the seller's understanding of) the license, and one can argue that it constitutes an embodiment of the exact license terms.

It doesn't matter whether you prefer to circumvent a particular filter that tests whether an installation is legal and would prefer to substitute one of your own making, Congress passed a law, and if you don't follow that law, you are in violation of the law (presumably that is obvious).

A question possibly suitable for Politics SE would be "why did Congress pass this particular anti-circumvention provision, what political facts gave rise to this section. From the LSE perspective, the law is the law, there is no why.