Question

Estimate the possible key length of the cipher text “TIGYXKLMPOUMEFRTIGBVTNJPGEGCL” generated by Vigenere Cipher, using Kasiski Test.

Answer 1

Kasiski examination focuses on determining the key size for a polyalphabetic substitution ciphers using keywords (such as for Vigenère cipher). With the keyword length, the ciphertext is lined in n columns (where n is the keyword length). Each column is then defined with a monoalphabetic substitution cipher, and where frequency analysis can be used to crack the key.

The Kasiski examination looks for repeated strings of characters in the ciphertext, and where these strings are three characters or more characters long. Overall the distances between these occurrences are then used to be multiples of the keyword length of the keyword.

TIGYXKLMPOUMEFRTIGBVTNJPGEGCL

Here the Repeated sequences is :
TIG

when we are analysing the repeated length:

3 letter:1
5 letter:1
15 letter:1

The Kasiski method predicts key sizes of: 3 5 15