A polyalphabetic cipher that shifts each letter by an amount determined by a repeating keyword. Because a single frequency analysis can't crack it the way it cracks Caesar, it was long nicknamed "the indecipherable cipher."
⚠️ Educational/puzzle tool only — do not use this for real security purposes.
The Vigenère cipher dates to the 16th century. It's often credited to Blaise de Vigenère, though the technique actually traces back further to work by Giovan Battista Bellaso. For centuries it was considered unbreakable, earning the nickname "le chiffre indéchiffrable" — the indecipherable cipher.
The keyword is repeated until it matches the length of the plaintext, and each plaintext letter is shifted by the alphabet position of the corresponding keyword letter (A=0, B=1, and so on). In effect, it's like running several different Caesar shifts across the message at once, instead of a single fixed shift.
The Vigenère cipher was eventually cracked in the 1800s through Kasiski examination — devised by Friedrich Kasiski — and independently by Charles Babbage, both of whom found repeated patterns in the ciphertext to deduce the keyword's length. Today it's of purely historical and educational interest and offers no real security by modern standards, so keep it to education and puzzles.