How do I find the key of the Vigenere cipher is if I already know the key length? Hi I am stuck in trying to crack the code of finding the key to decrypt a huge message. I have the frequencies of the letters:

A 121
B 0
C 10
D 60
E 39
F 100
G 123
H 19
I 0
J 74
K 87
L 119
M 35
N 8
O 30
P 2
Q 27
R 0
S 135
T 26
U 47
V 47
W 140
X 24
Y 29
Z 78
A 121
B 0
C 10
D 60
E 39
F 100
G 123
H 19
I 0
J 74
K 87
L 119
M 35
N 8
O 30
P 2
Q 27
R 0
S 135
T 26
U 47
V 47
W 140
X 24
Y 29
Z 78


So far I figured out that the first letter is an "S" since W = 22 so 22-4 = 18 which is "S"
If you know the key length of a Vigenere cipher, you should group the letters in the given huge message by each of the key letters, then compute the frequencies for each group separately. If I understand right, you are doing a total frequency analysis, which is of no use.