https://craphound.com/msftdrm.txt

In DRM, the attacker is also the recipient. It’s not Alice and Bob and Carol, it’s just Alice and Bob. Alice sells Bob a DVD. She sells Bob a DVD player. The DVD has a movie on it—say, Pirates of the Caribbean—and it’s enciphered with an algorithm called CSS—Content Scrambling System. The DVD player has a CSS un-scrambler.

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… Alice wants Bob to buy Pirates of the Caribbean from her. Bob will only buy Pirates of the Caribbean if he can descramble the CSS-encrypted VOB—video object—on his DVD player. Otherwise, the disc is only useful to Bob as a drinks-coaster. So Alice has to provide Bob—the attacker—with the key, the cipher and the ciphertext.