Reference
The Vigenere Cipher Explained
The Vigenere cipher improves on Caesar by using a keyword so that each letter is shifted by a different amount. Repeat the keyword across the message, and shift each letter by the position of the matching key letter. For centuries it was called the indecipherable cipher.
How the keyword works
Write the keyword repeatedly above the message. Each key letter gives a shift: A means shift 0, B means 1, and so on. Shift each message letter by its key letter's value, wrapping around the alphabet.
Because the shift changes from letter to letter, the same plaintext letter can encrypt to different ciphertext letters. That hides the letter-frequency patterns that give a simple Caesar cipher away.
Why it was finally broken
The weakness is that the keyword repeats. If an attacker finds the key length, they can split the message into several simple Caesar ciphers and solve each with frequency analysis.
Charles Babbage and Friedrich Kasiski worked out how to find the key length in the 19th century by spotting repeated patterns in the ciphertext. A key as long as the message, never reused, is the one version that is truly unbreakable.
Frequently asked questions
Why is the Vigenere cipher stronger than Caesar?
It uses a keyword so the shift changes for every letter, which hides the letter-frequency clues that let people crack a single fixed Caesar shift.
How was the Vigenere cipher cracked?
By finding the length of the repeating keyword. Once the key length is known, the message splits into several Caesar ciphers that each fall to frequency analysis.