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Genetic Algorithms are search algorithm. They use a metaphor where an optimization problem takes the place of an environment and feasible solutions are considered as individuals living in that environment.

This paper focuses on using GAs to cryptanalyse a transposition cipher. A certain method is used to find the possible key length. It is shown that such algorithm could be used to discover the key for transposition cipher. The focus is to be on a bi-and Tri-gram frequency of letters. The frequency analysis is used as an essential factor in the objective function. The length of text was 3000 letters. The population size are 10, 20, 30, 40, and 50. The key size is 10 letters. Two equations are used to find optimal soulution. The first one depends on the letter frequency of bigram, and the second one depends on score table. The result clarifies that the second equation gave the optimal solution for the population size 30, and 40, where the number of correct letters was 10 out of 10 letters.