The Kasiski(s) command computes the so-called Kasiski-test for the string s. This is defined to be the least common multiple of the lengths of repeated substrings of s.

In elementary cryptanalysis, it is often used in conjunction with the index of coincidence to attempt to determine the number of alphabets used to encipher a plain text with a polyalphabetic substitution cipher.

All of the StringTools package commands treat strings as (null-terminated) sequences of $8$-bit (ASCII) characters.  Thus, there is no support for multibyte character encodings, such as unicode encodings.

$\mathrm{with}\left(\mathrm{StringTools}\right)\:$

$\mathrm{Kasiski}\left(''abcde''\right)$

$0$

$\mathrm{Kasiski}\left(''abcdeabcdabc''\right)$

$1$