Kasiski Test

This method was published by Prussian infantry officer and cryptographer Friedrich Wilhelm Kasiski in 1863. He noted that where a group of plaintext letters was repeated and lay under the same position in the keyword, the cipher text would also be repeated.

This can be exploited by examining repeated text and checking the distances between repeats. The most common factor is usually the true period, but one should allow for the factors of the period also being frequent. Thus, if the period is 12, then the factors 6, 4 and 3 will also be common. To address this, a weighted factor is used which is the factor multiplied by the frequency of that factor. The factor with the highest weighted factor is a good contender for the cipher period.