Cory Doctorow explained it very nicely (in his talk to the Microsoft Research group to be found here [craphound.com]):The original text by Cory Doctorow which this post quotes is well worth reading.When will they ever understand?Cryptography - secret writing - is the practice of keeping secrets. It involves three parties: a sender, a receiver and an attacker [...]. We usually call these people Alice, Bob and Carol. [A few explanations of cipher, ciphertext and key] In DRM, the attacker is *also the recipient*. It's not Alice and Bob and Carol, it's just Alice and Bob. So Alice has to provide Bob - the attacker - with the key, the cipher and the ciphertext.
DRM systems are usually broken in minutes, sometimes days. Rarely, months. It's not because the people who think them up are stupid. It's not because the people who break them are smart. It's not because there's a flaw in the algorithms. At the end of the day, all DRM systems share a common vulnerability: they provide their attackers with ciphertext, the cipher and the key. At this point, the secret isn't a secret anymore.
Among the replies, also note this one:
The various media concerns are trying to get the TV/DVD player/whatever to be Bob, while casting the consumer in the role of Carol. That's what all the fuss is about.