1
$\begingroup$

I was learning about the finding the key length reading the following web site... http://practicalcryptography.com/cryptanalysis/stochastic-searching/cryptanalysis-vigenere-cipher/

and I really don't understand some part of the example shown in the web. They assume the key length is 2 and extract the two sequence 1,3,5,7.... and 2,4,6,8... and from these two sequences they calculate the I.C.

I wasn't sure how they calculate the I.C. for each sequence. Do they find the frequency count of each letter from a to z, and calculate the I.C using the following formula?

Formula for the index of coincidence

$\endgroup$
1
$\begingroup$

As explained on the link you posted, the Vigenere cipher with a key on length $n$ encrypts every $n$-th symbol with the same key under the Caesar cipher. So to calculate the IC you should take all the $n$ sub-sequences separately: $\{1, 1+n, \dots, 1+kn, \dots\}$, $\{2, 2+n, \dots, 2+kn, \dots\}$ and so on and compute the IC for every sub-sequence.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.