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Broadcast Encryption

key device client management

Definition: In one-way communication system, broadcast encryption is intended to provide a means for two parties not known to each other, to communicate a cryptographic key for content protection and other applications.

Broadcast encryption was first proposed by Fiat, et. al, of IBM. The initial goal was to allow a central broadcast site to broadcast secure transmissions to an arbitrary set of recipients while minimizing key management related transmissions. If we use a naive approach where each client device is given its own key and an individually encrypted message is transmitted to all legitimate client devices, a very long transmission (the number of legitimate devices times the length of the message) is required. On the other hand, if we group legitimate devices into groups and each legitimate device is given all the keys corresponding to the group it belongs, every legitimate device needs to store a lot of keys. Motivated to create practical solutions, Fiat, et al, invented Broadcast encryption where efficiency are achieved at both transmission and client device storage. For example, one of the schemes they proposed requires the server to broadcast O(k2log2klogn) messages and every client device to store O(klogklogn) keys with a universe of size n and a resilient capability to any coalition of k users.

In recent years, Broadcast encryption found its value not only in broadcast applications but also packaged media content applications. This is because packaged media, from manufacture to consumers possesses the same one-way nature as broadcasting.

One key difference between traditional cryptography and Broadcast encryption is that ‘classic security uses a cryptographic handshake at the link level to prevent eavesdropping on the communication between two secure boxes while Broadcast encryption hides device keys deeper in the software, near the point of content consumption’. Several Broadcast encryption schemes have been proposed. They use simple symmetric encryptions instead of public key encryptions, which significantly reduce the computational complexity at both the server and client devices. The basic idea of all proposed schemes is similar. They employ key management block that is available in the one-way communication to communicate a management key for message/content decryption. At the server side, all protected content is encrypted with keys based on the management key. At the client side, the key management block is processed by the client device to generate the management key for content decryption. For instance, in a device key matrix based approach, the key management block, maybe generated by encrypting the management key multiple times using each different device key, is communicated to the client device in the one-way communication. A legitimate client device that knows a device key’s position in the matrix can decrypt the value found at that position. Readers can reference the article listed in the References section for more details on Broadcast encryption.

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