A short key goes a long way.
Write the keyword under your message, repeating it as needed. Skip spaces and punctuation so only letters use a key character.
ATTACK AT DAWN
LEMONL EM ONLEA LITTLE KEY. A LOT OF POSSIBILITIES.
Secrets, letter by letter.
Meet the Vigenère cipher. A repeating keyword shifts each letter of your message. Try it, then follow the transformation.
01 Make a secret
A–Z letters are transformed. Spaces, numbers, and punctuation stay put.
Use letters A–Z. The key repeats to match your message.
Enter a key using only letters A–Z.
ENCRYPTED MESSAGE
Calculated as you type
02 Follow the letters
Select any letter to explore itTHE CURRENT TRANSFORMATION
AMESSAGE
+LKEY
=LCIPHER
The Vigenère square
26 shifts × 26 lettersMESSAGE LETTER →
Message columnKey rowCipher letter
03 The deciphering desk
Find the key.
Reveal the message.
Start with a possible key length, try a frequency guess, then fine-tune each letter.
01 / FIND THE RHYTHM
How long is the key?
Look for peaks. Each bar measures how much the letter patterns resemble ordinary language. Select a length to try it.
Average coincidence (IC)
Scroll to compare all lengths →
Add at least two letters to see key-length clues.
English ≈ 0.065Random ≈ 0.038
How to read these clues
IC measures how often two letters in a column match. The chart averages it across all columns for each length from 1–20. Bars are capped at 0.10; hover or focus to read the exact score. Use the length menu for keys up to 64 letters.
Multiples of the real length can also have high scores. Try a peak and its factors. Short samples are unreliable; English frequency guesses work best with plenty of text.
02 / FINE-TUNE A LETTER
Find the shift in each column.
The repeating key uses the same shift at regular intervals. Gather those letters into a column, undo a possible shift, then compare the result with English.
Cipher
Your plaintextTypical English
Share of letters in this column
Scroll to see the full alphabet →
Changing the key letter moves the purple bars around the alphabet; the green English reference stays fixed. Look for a similar overall shape, then check the plaintext preview for readable words.
How are the shifts ranked?
We try all 26 backward shifts and count the resulting letters. The χ² (chi-squared) score measures how far those counts are from typical English. Lower means a closer frequency match, not a probability that the key is correct.
“Suggest a key” picks the lowest-scoring shift separately for each key position. Short samples and an incorrect key length can give misleading matches. Try other highly ranked letters, and use the words in the full preview to decide.
Show exact letter counts
| Letter | Count | Share | Comparison | English |
|---|
Show this column’s ciphertext
03 / FOLLOW THE PATTERNS
Some letters come back.
Repeated sequences can reveal the key’s rhythm. Look for lengths that divide the gaps between them.
| Sequence | Count | Gaps | Factors |
|---|
About the Kasiski method
Gaps count only A–Z letters, using consecutive occurrences and including overlaps. Factors from 2–64 divide every gap for that sequence. The table shows up to 20 sequences and 12 gaps each.
A repeat may be a coincidence. Combine these clues with the key-length chart and the words in your preview.
A starting point for English text. Read the words and revise your guesses.
04 The idea behind the secret
A different shift for every letterEach key letter is a number.
A means shift by 0, B by 1, and so on. Add that shift to encrypt; subtract it to decrypt. Wrap around whenever you pass the end of the alphabet.
Y + E → 24 + 4 → 28 → CThe same letter can change.
With the key LEMON, the first A in ATTACK becomes L, while the next A becomes O. Different key letters create different shifts.
A classic for learning cryptography. Repeating keys leave patterns, so this cipher is not secure for real secrets.