Cryptanalysis of the RC4 Stream Cipher using Evolutionary ...

2.3.1 Chromosome Representation
Potential solutions can be represented in several ways but the most common representation
is through binary strings where each bit represents a characteristic flag. The other common
representation is through a numerical value as the bit string, whether it is an integer or
real number.
For the S register of RC4 each candidate solution will be represented as
2 n sets of n-bit binary strings. For example, a candidate solution is represented as C i =
(w 1 , w 2 , . . . , w 2 n) where w i = {0, 1, . . . , 2 n − 1} and each value of w i is unique due to its
permutation property. All operations documented are performed on a whole n-bit word w i
or multiple words and never on a subset of w i to maintain the chromosomes permutation
property.
2.3.2 Selection
Selection is the process of probabilistically choosing the best candidate solutions for the next
generation. This is done by evaluating the fitness of each chromosome in order to assign
a better probability to the more fit solutions to enter the next generation.
This Thesis
will use two popular methods as a way to contrast their influence on the best particular
approach for a permutation problem. The methods are fitness proportional representation
and tournament selection which will be elaborated in Chapter 4.
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