Encrypting with a password

Caesar’s cipher has the flaw of applying the same shift to all letters. Vigenère corrects this: the shift changes for each letter, following a repeated key.

The method

The key is written beneath the message, repeated as follows:

clair   M  A  T  H  S
clé     C  L  E  C  L

Each letter in the key corresponds to a shift: A is 0, B is 1, C is 2, … L is 11, E is 4.

M + C = M + 2  → O
A + L = A + 11 → L
T + E = T + 4  → X
H + C = H + 2  → J
S + L = S + 11 → D

The ciphertext is OLXJD.

What this changes

Look at the two As in the word ANANAS, encrypted with the key CLE:

clair   A  N  A  N  A  S
clé     C  L  E  C  L  E
chiffré C  Y  E  P  L  W

The first A becomes C, the third becomes E, and the fifth becomes L. The same plaintext letter yields different ciphertext letters.

This is exactly what was missing from substitution ciphers: frequency analysis no longer works. Counting the letters in the ciphertext is no longer meaningful, as each E in the plaintext has been spread across several ciphertext letters.

This is known as polyalphabetic ciphering: several substitution alphabets used alternately.

The cipher was considered unbreakable for so long that it was known as ‘the indecipherable cipher’. It held up for around three centuries.