Broadcast encryption
Encyclopedia
Broadcast encryption is the cryptographic
problem of encrypting
broadcast
content (e.g. TV programs) in such a way that only qualified users (e.g. subscribers who've paid their fees) can decrypt the content. The challenge arises from the requirement that unsubscription of some users should not affect the remaining users. The problem is further complicated by the fact that there is typically more than one data stream ("channel"), each of which may have a different set of subscribers. Several solutions exist offering various tradeoffs between the increase in the size of the broadcast, the number 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.
The problem of rogue users sharing their decryption keys with unqualified users is mathematically insoluble. Traitor tracing
algorithm
s 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 box
es with tamper-resistant
smart card
s that impose physical restraints on a user learning their own decryption keys. Some schemes, such as AACS
allow the provider to eliminate any desired subset of users from being able to use the service by arranging keys in a binary tree
. Each item has a branch from a node to the root whose keys it does not know. If the master secret is encrypted with one of those keys it will not be able to be decrypted. Blu-ray and HD DVD
both use this scheme.
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
problem of encrypting
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
content (e.g. TV programs) in such a way that only qualified users (e.g. subscribers who've paid their fees) can decrypt the content. The challenge arises from the requirement that unsubscription of some users should not affect the remaining users. The problem is further complicated by the fact that there is typically more than one data stream ("channel"), each of which may have a different set of subscribers. Several solutions exist offering various tradeoffs between the increase in the size of the broadcast, the number 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.
The problem of rogue users sharing their decryption keys with unqualified users is mathematically insoluble. Traitor tracing
Traitor tracing
Traitor tracing is a copyright infringement detection system which works by tracing the source of leaked files rather than by direct copy protection. The method is that the distributor adds a unique value to each copy given out...
algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s 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
Pay TV
Pay television, premium television, or premium channels refers to subscription-based television services, usually provided by both analog and digital cable and satellite, but also increasingly via digital terrestrial and internet television...
systems often employ set-top box
Set-top box
A set-top box or set-top unit is an information appliance device that generally contains a tuner and connects to a television set and an external source of signal, turning the signal into content which is then displayed on the television screen or other display device.-History:Before the...
es with tamper-resistant
Tamper resistance
Tamper resistance is resistance to tampering by either the normal users of a product, package, or system or others with physical access to it. There are many reasons for employing tamper resistance....
smart card
Smart card
A smart card, chip card, or integrated circuit card , is any pocket-sized card with embedded integrated circuits. A smart card or microprocessor cards contain volatile memory and microprocessor components. The card is made of plastic, generally polyvinyl chloride, but sometimes acrylonitrile...
s that impose physical restraints on a user learning their own decryption keys. Some schemes, such as AACS
Advanced Access Content System
The Advanced Access Content System is a standard for content distribution and digital rights management, intended to restrict access to and copying of the "next generation" of optical discs and DVDs. The specification was publicly released in April 2005 and the standard has been adopted as the...
allow the provider to eliminate any desired subset of users from being able to use the service by arranging keys in a binary tree
Binary tree
In computer science, a binary tree is a tree data structure in which each node has at most two child nodes, usually distinguished as "left" and "right". Nodes with children are parent nodes, and child nodes may contain references to their parents. Outside the tree, there is often a reference to...
. Each item has a branch from a node to the root whose keys it does not know. If the master secret is encrypted with one of those keys it will not be able to be decrypted. Blu-ray and HD DVD
HD DVD
HD DVD is a discontinued high-density optical disc format for storing data and high-definition video.Supported principally by Toshiba, HD DVD was envisioned to be the successor to the standard DVD format...
both use this scheme.