Encryption for Digital Content

A broadcast channel enables a sender to reach many receivers in a very effective way. Broadcasting, due to its very nature, leaves little room for controlling the list of recipients N —once a message is put on the channel any listening party can obtain it. This may very well be against the objectives of the sender. In such case, encryption comes in mind as a potential way to solve the problem: it can be employed to deny eavesdroppers free access to the content that is broadcasted. Nevertheless, the use of encryption raises the issue of how to do key management. Enabled receivers should be capable of descrambling the message while eavesdroppers should just perceive it as noise. It follows that receivers that are enabled for reception should have access to the decryption key, while any other party should not. The major problem that springs up in this scenario is that receivers might get corrupted and thus become cooperative with the adversary. As a result one cannot hope that a party that owns a key will not use it to the fullest extend possible, i.e., for as long as such key allows descrambling which can be the moment that a global rekey operation takes place. Moreover, such a key can even be shared with more than a single listening party and thus enable the reception of the transmission for a multitude of rogue receivers. If a traditional encryption scheme is used then a single corrupted receiver is enough to bring forth such undesired effects. The subject of this chapter, broadcast encryption deals with solving the above problem in an effective way.

A three word description for what traitor tracing aims to achieve is key leakage deterrence. Arguably the single most important problem in the application of cryptography is key management. Keys that are lost imply loss of data and keys that are exposed imply loss of privacy. In the context of digital content distribution the problem is heavily exacerbated by the fact that cryptographic keys reside within a possibly adversarial environment.