Cipher Design Implications
Given the possibly extreme severity of a correlation attack's impact on a stream cipher's security, it should be considered essential to test a candidate Boolean combination function for correlation immunity before deciding to use it in a stream cipher. However, it is important to note that high correlation immunity is a necessary but not sufficient condition for a Boolean function to be appropriate for use in a keystream generator. There are other issues to consider, e.g. whether or not the function is balanced - whether it outputs as many or roughly as many 1's as it does 0's when all possible inputs are considered.
Research has been conducted into methods for easily generating Boolean functions of a given size which are guaranteed to have at least some particular order of correlation immunity. This research has uncovered links between correlation immune Boolean functions and error correcting codes.
Read more about this topic: Correlation Attack
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