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Vigenère Cipher Decoder

Decode a Vigenère cipher when you have the keyword: this page opens in decode mode with Elew aa tf tks kdk buwzyl aw aevuijvp. in the box and SHADOW as the key, which reads Meet me by the old bridge at midnight. Replace both with your own and the answer changes as you type. Spaces and punctuation come through untouched and do not use up a key letter, which is the rule that decides whether a decode agrees with the textbook or quietly disagrees with it.

Plain textVigenère, key SHADOW
Meet me by the old bridge at midnight.
Cipher
Mode
Key
Autokey

Each letter of the key shifts one letter of the message, and the key repeats for as long as the message lasts. Letters A to Z shift; spaces, punctuation, digits, accented letters and emoji pass through untouched, and they do not use up a key letter.

The tabula recta

The row is the key letter, the column is the plaintext letter, and the cell where they meet is the ciphertext letter. Vigenère walks the key one letter at a time, taking a row per letter. The rows your key uses are highlighted.

Key letterABCDEFGHIJKLMNOPQRSTUVWXYZ
AABCDEFGHIJKLMNOPQRSTUVWXYZ
BBCDEFGHIJKLMNOPQRSTUVWXYZA
CCDEFGHIJKLMNOPQRSTUVWXYZAB
DDEFGHIJKLMNOPQRSTUVWXYZABC
EEFGHIJKLMNOPQRSTUVWXYZABCD
FFGHIJKLMNOPQRSTUVWXYZABCDE
GGHIJKLMNOPQRSTUVWXYZABCDEF
HHIJKLMNOPQRSTUVWXYZABCDEFG
IIJKLMNOPQRSTUVWXYZABCDEFGH
JJKLMNOPQRSTUVWXYZABCDEFGHI
KKLMNOPQRSTUVWXYZABCDEFGHIJ
LLMNOPQRSTUVWXYZABCDEFGHIJK
MMNOPQRSTUVWXYZABCDEFGHIJKL
NNOPQRSTUVWXYZABCDEFGHIJKLM
OOPQRSTUVWXYZABCDEFGHIJKLMN
PPQRSTUVWXYZABCDEFGHIJKLMNO
QQRSTUVWXYZABCDEFGHIJKLMNOP
RRSTUVWXYZABCDEFGHIJKLMNOPQ
SSTUVWXYZABCDEFGHIJKLMNOPQR
TTUVWXYZABCDEFGHIJKLMNOPQRS
UUVWXYZABCDEFGHIJKLMNOPQRST
VVWXYZABCDEFGHIJKLMNOPQRSTU
WWXYZABCDEFGHIJKLMNOPQRSTUV
XXYZABCDEFGHIJKLMNOPQRSTUVW
YYZABCDEFGHIJKLMNOPQRSTUVWX
ZZABCDEFGHIJKLMNOPQRSTUVWXY
These are puzzle ciphers, not security. Vigenère went unbroken for three centuries and then stopped: repeated patterns in the ciphertext give away the key length, and each position then falls to the same frequency count this page runs on Caesar. Use them for puzzles, ciphers in games and homework. Anything that actually needs to stay private wants modern encryption.
Accuracy. Exact for the classical ciphers as defined: letters shift, everything else passes through and, in Vigenère, does not consume a key letter. The Caesar cracker ranks shifts by English letter frequency and is a guess, shown as one.

Common questions

How do I decode a Vigenère cipher?
Put the ciphertext in the left box, the keyword in the Key field, and leave the mode on Decode. The key is repeated across the message one letter per letter, and each key letter shifts its letter back instead of forward. Elew aa tf tks kdk buwzyl aw aevuijvp. under SHADOW comes back as Meet me by the old bridge at midnight.
Can I decode a Vigenère cipher without the key?
Not here, and not quickly anywhere. Breaking Vigenère without the key means finding the key length first, usually from repeated groups in the ciphertext, then solving each position as its own single-shift cipher. This tool cracks the single-shift case: switch the cipher to Caesar and the Crack mode ranks all 25 shifts by English letter frequency. For Vigenère it needs the key.
What happens if the key is wrong?
Nothing complains. Every key produces a result, so a wrong one gives you letters rather than an error: the same ciphertext under LEMON reads Thsi np pt fxh grw ojsnky ps oqijexhc. A decode that comes out as words is how you know the key was right.
Does the key have to be one word?
It has to be letters. Spaces, digits and punctuation are not part of a Vigenère key, so the field asks you to take them out and tells you what the key becomes without them. Case does not matter: lemon and LEMON are the same key.
What is autokey, and do I need it?
Autokey is the variant Vigenère himself published: after the keyword runs out, the message itself continues the key instead of the keyword repeating. Half the puzzles labelled Vigenère are this one. The switch above decodes it, recovering each plaintext letter and feeding it straight back in as the next key letter. If a decode with the right keyword goes to nonsense a few words in, try it.
Is the ciphertext sent anywhere?
No. The cipher runs in your browser and nothing is uploaded. This page always opens on its worked example, the SHADOW ciphertext, rather than on what you last typed: the cipher page is the one that keeps your message and key, in this browser only. Clear empties the box, puts the settings back and removes what was stored.

Exact for the classical ciphers as defined: letters shift, everything else passes through and, in Vigenère, does not consume a key letter. The Caesar cracker ranks shifts by English letter frequency and is a guess, shown as one.