🔐 The Vigenère Cipher

The Vigenère Cipher is a polyalphabetic substitution cipher that uses a keyword to encrypt text. It was considered unbreakable for over 300 years!

How It Works

Unlike the Caesar cipher which uses a single shift, the Vigenère cipher uses a keyword where each letter determines a different shift:

A = shift 0, B = shift 1, C = shift 2, ... Z = shift 25
The keyword is repeated to match the length of the plaintext
Each plaintext letter is shifted by its corresponding keyword letter

Example

Plaintext: ATTACKATDAWN

Keyword: LEMON

Plaintext: A T T A C K A T D A W N Keyword: L E M O N L E M O N L E Shifts: 11 4 12 14 13 11 4 12 14 13 11 4 Ciphertext: L X F O P V E F R N H R Result: LXFOPVEFRNHR

Encryption Formula

Ciphertext = (Plaintext + Key) mod 26 Example: A (0) + L (11) = L (11) T (19) + E (4) = X (23)

How to Crack It

Find the Key Length: Use the Kasiski examination or Index of Coincidence to find repeating patterns
Split into Groups: Once you know the key length, split the ciphertext into groups
Frequency Analysis: Each group is essentially a Caesar cipher - use frequency analysis on each
Combine: Put the individual Caesar shifts together to find the keyword

Kasiski Examination

Look for repeated sequences in the ciphertext. The distance between repetitions is likely a multiple of the key length.

Tips

Longer keywords are harder to crack
If the keyword is as long as the message and random, it becomes a One-Time Pad (unbreakable!)
Common words like "THE" can help identify key length
Online tools can help calculate the Index of Coincidence