Vigenère Cipher Tool
NewEncrypt or decrypt text with the classic Vigenère polyalphabetic cipher using a repeating keyword, with a letter-by-letter breakdown.
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Unlike a Caesar cipher's single fixed shift, the Vigenère cipher shifts each letter by a different amount, determined by the repeating keyword — this makes simple frequency analysis far less effective. Only letters A-Z/a-z are shifted; digits, punctuation, and spaces pass through unchanged and don't consume a keyword letter.
Frequently Asked Questions
How is the Vigenère cipher different from a Caesar cipher?
A Caesar cipher shifts every letter by the same fixed amount, which makes it vulnerable to simple frequency analysis; the Vigenère cipher instead shifts each letter by a different amount determined by the corresponding letter of a repeating keyword, so the same plaintext letter can encrypt to different ciphertext letters depending on position.
What happens to spaces, digits, and punctuation in the input?
They pass through unchanged and, importantly, don't consume a letter from the keyword — only actual letters (A-Z/a-z) are shifted and advance the keyword position, matching the traditional definition of the cipher.
Is this cipher actually secure for real-world use?
No — the Vigenère cipher was considered unbreakable for centuries but was fully broken in the 1800s using the Kasiski examination and related frequency-analysis techniques once the keyword length is guessed; it's taught today for historical/educational value and puzzle-solving, not for protecting real secrets.
How does decryption actually work?
Decryption reverses the same per-letter shift: instead of adding the keyword letter's shift to the ciphertext letter, it subtracts it (wrapping around the alphabet), which recovers the original plaintext letter exactly.
What does the letter-by-letter breakdown show?
Three aligned rows — the input text, the repeating keyword letter used for each position, and the resulting output letter — so you can see exactly which keyword letter produced each shift, which is especially useful when learning or verifying the cipher by hand.