Broadcast encryption
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Broadcast encryption is the cryptographic problem of encrypting broadcast content (e.g. TV programs or data on DVDs) in such a way that only qualified users (e.g. subscribers who have paid their fees or DVD players conforming to a specification) can decrypt the content. The challenge arises from the requirement that the set of qualified users can change in each broadcast emission, and therefore revocation of some users should not affect the remaining users. Several solutions exist offering various tradeoffs between the increase in the size of the broadcast, the size of keys that each user needs to store, and the feasibility of an unqualified user or a collusion of unqualified users being able to decrypt the content.
Schemes in use include CSS for DVD encryption and AACS for HD DVD and Blu-ray Disc encryption.
The problem of rogue users sharing their decryption keys or the decrypted content with unqualified users is mathematically insoluble. Traitor tracing algorithms aim to minimize the damage by retroactively identifying the user or users who leaked their keys, so that punitive measures, legal or otherwise, may be undertaken. In practice, pay TV systems often employ set-top boxes with tamper-resistant smart cards that impose physical restraints on a user learning their own decryption keys. Some broadcast encryption schemes, such as AACS, also provide tracing capabilities.[1]
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