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Chapter 2
Theory
This chapter contains an overview of the broadcast encryption area in general, and
more detailed descriptions of the algorithms implemented in this system.
2.1
Introduction
Broadcasting means transmitting information through a medium that is accessible
to multiple receivers. A radio transmission for instance uses air as a medium,
and everyone with a radio receiver is able to listen to the broadcast. Usually
this communication is one-way, meaning that the receivers are not able to send
anything back to the broadcast center.
In some applications it is desirable to secure the content of the broadcasted
message so that only the authorized users are able to read it. In broadcast encryption theory these users are said to be privileged, and the non authorized users are
said to be revoked. The terms receivers and users are sometimes used interchangeably. The difference is that all the receivers can access the broadcast message, but
only the privileged users can access the content of the message.
The broadcast message is usually divided into a header and a body part. The
body contains the protected content and the header contains information needed
to access the content (key material and user memberships). The header is the
most important part when analysing these algorithms.
The most simple broadcast encryption scheme would be to encrypt the message
once for each privileged user and then broadcast all encrypted messages. This
is obviously a very inefficient scheme in terms of processing time and broadcast
message size. The aim of all intelligent schemes is to reduce the processing time at
both the broadcast center and at the receivers, to reduce the broadcast message
size and to reduce the storage size at the receivers. The broadcast message size
is dependent on the number of encrypted messages it contains, but also on the
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