Let me explain with using a concrete
example.
First, this is the list of the Caesar
Ciphers used in Vigenere
Cipher.
Each alphabet represent one of the Caesar
Ciphers.
For example, the alphabet “a” represents
the Caesar Cipher with zero
shifts, that means that the alphabets are not
changed at all.
The alphabet “b” represents the Caesar
Cipher with one shift, that
means that “a” is substituted with “b”, “b” is
substituted with “c”, and so son.
The alphabet “c” represents the Caesar
Cipher with two shifts like this, and so on.
Now I will show you how the sentence
“this is a pen” is ciphered with the key “age”.
The first line represents the plain text
“this is a pen”.
The second line is the repetetion of
the key.
Then the cipher text is obtained by
substituting each alphabet in the plain text
by using the Caesar Cipher
designated just below the character.
As the conclusion, we obtain the cipher text: tnmsowavin
It is known that Vigenere Cipher is the ultimate cipher, that is, it is impossible
to break it if it is properly used, where "properly used" means that you have
to use a very long key consisting of random alphabets.
