** Previous Level**: Level 4

## Login

SSH: `ssh krypton5@krypton.labs.overthewire.org -p 2231`

## Task

Frequency analysis can break a known key length as well. Lets try one last polyalphabetic cipher, but this time the key length is unknown.

## A little bit of Theory

The **Kasiski examination** is an attack on polyalphabetic cipher that makes it possible to guess the key length. The method looks for repeated sequences and their offsets/distances. The greatest common divisor of the offsets would most likely be the key length.

## Solution

This task is similar to the previous one, just that the key length is also unknown. Which means, we have to guess it.

- Just Brute-Force/guess it. Start with key length = 2 and see if the Vigenère Cipher breaker gets a meaningful (English) solution.
- I wanted to see if there is also another, better method to guess the key. I found the Kasiski examination.

Our trusted Vigenère Cipher breaker from the last task offers this method as well. The result is that our key is most likely of length 3, 6 or 9. One can see that 6 and 9 are multiples of 3, so the repetition of characters and sequences happened in multiples of 3, given the hint for the key length.

Now that a possible key length has been narrowed down, we can proceed like in the previous level.
This leads us to the correct key length of 9 and the key.
When decrypting the `krypton5`

file, we get the password for the next level.