Good stream ciphers are hard to design

Cipher systems are commonly subdivided into block ciphers and stream ciphers. In the application, block ciphers require, as their name indicates, large blocksizes. Contrastingly, stream ciphers can operate on data units as small as a bit or a character, a fact that has greatly contributed to their popularity. In this paper a survey of the different approaches to the design of stream cipher systems is presented. Examples are used to illustrate the results that can be obtained.