Vignere Cipher

This is like the Caesar Cipher, but instead of adding the same shift to each letter position, we add a variable one.

Method

1. Convert keyword to the position in the alphabet. We index by $0$, so A is $0$.
2. Add it to the ciphertext and mod 26.

For decryption, we subtract.

Example 1

Keyword: DOG Plaintext: SWIM

So, DOG -> 3 14 6 and SWIM -> 18 22 8 12, so

18 + 3  = 21 
22 + 14 = 36 -> 14
8  + 6  = 14
12 + 3  = 15

Example 2

Keyword: AND Plaintext: Six Meals

A = 0
N = 13
D = 3

So,

S -> 18 + 0  = 18
I -> 8  + 13 = 21
X -> 23 + 3  = 26 mod> 0
M -> 12 + 0  = 12
E -> 4  + 13 = 17
A -> 0  + 3  = 3
L -> 11 + 0  = 11
S -> 18 + 13 = 31 mod> 5

And so the ciphertext is SVAMRDLF.