# Decryption of Vigenère cipher by hand using Kasiski method

My task is to decrypt the following message, which was encrypted with a Vigenère cipher of length 4:

BCRRBCQORHKEPSLSLCWRWXXDESPEZMPYQWCEBCBOSFHCIZHSQWV HCBRWRVLNEGDRCKRRQS.


I used the Kasiski method but I must be doing something wrong.

The first charter in the sets of 4 the letters was B=3 and Q=3 but if B=E or Q=E then then shift is incorrect.

The second character in the set of 4 is C if C=E then it does not work,
The third character in the set of 4 is R if R=E then it does not work,
The fourth character in the set of 4 is E if E=E then it works.

How am I going to figure out the first-, second-, and third shift using the Kasiski method?

Keep in mind I will have to do this on paper during a timed test.

I know the code is YODA and the plaintext is:

DO OR DO NOT THERE IS NO TRY JUDGE ME BY MY SIZE DO YOU RECKLESS IS HE NOW THINGS ARE WORSE

• Not sure this is homework. $K=YODA = (24,14,3,0)$. In order to decrypt you need $C_i-K_j \bmod 26$ where $j = i \bmod 4$. The first one: $B-Y = 2 - 24 = -22 = 4 = D$... – kelalaka Jan 24 '20 at 0:43
• @kelalaka You are way over my head. I will have a test just like this but with different Message and different key length. I need to be able to do this somewhat quickly. I know I am asking a lot but I have beaten my head for days and no one really explaining it well on the internet. I feel like I did the method right but E= did not equated to any of my frequently used numbers except the last position . I am not supposed to know the key. Can you explain in detail and give and example. – jason Jan 24 '20 at 1:31
• Are you given the length of the keyword? The question is not written in a good way. You need to use the paper with a soft pen. You need x (length of the keyword) frequency table though not all of it so that you can place the common letters in each group. – kelalaka Jan 24 '20 at 17:18