Vigenere Cipher

Reference

What is the Vigenere cipher?

The Vigenere cipher is a polyalphabetic substitution cipher that uses a keyword to shift each letter by a different amount. Unlike the Caesar cipher which uses a single fixed shift, the Vigenere cipher cycles through multiple shifts based on the letters of the keyword. For example, with keyword "KEY", the first letter shifts by 10 (K), the second by 4 (E), the third by 24 (Y), then the pattern repeats. This makes frequency analysis much harder than with simple substitution ciphers.

How it works

Keyword expansion — the keyword is repeated to match the length of the plaintext (e.g., "KEY" becomes "KEYKEYKEY…"). Encoding — each plaintext letter is shifted forward by the corresponding keyword letter's position (A=0, B=1, …, Z=25). Decoding — each ciphertext letter is shifted backward by the keyword letter's position. Non-letters — digits, spaces, and punctuation pass through unchanged and do not advance the keyword position. Tabula recta — historically, a 26×26 grid of shifted alphabets was used as a lookup table for encoding and decoding.

Common use cases

Cryptography education — learn about polyalphabetic ciphers and why they are stronger than monoalphabetic ones. CTF competitions — Vigenere ciphers are a staple of capture-the-flag and puzzle challenges. Historical study — understand the cipher that was considered "unbreakable" for 300 years. Text obfuscation — obscure messages with a shared secret keyword. Cipher analysis — practice Kasiski examination and index of coincidence techniques.

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