The Vigenère cipher is a polyalphabetic substitution scheme published by Blaise de Vigenère in the 16th century. Instead of one fixed shift like Caesar, it uses a repeating keyword: each plaintext letter is shifted by the alphabetical position of the next keyword letter, cycling through the keyword as the message is encoded. For three centuries, it was nicknamed "le chiffre indéchiffrable" (the indecipherable cipher), until Charles Babbage and Friedrich Kasiski independently broke it.
For plaintext ATTACK with keyword KEY:
Plain : A T T A C K
Key : K E Y K E Y
Cipher: K X R K G I
Each plain letter is shifted forward by the alphabetical value of the corresponding key letter (A=0, K=10, E=4, Y=24, ...). To decrypt, shift backward by the same amount.
Type your keyword guess into the box above the ciphertext. The display below updates live: each letter shows the ciphertext (gold), the keyword letter applied to that position (small), and your decrypted plaintext (parchment). Read the bottom row aloud as you adjust the keyword. When it forms English, you have it.
The cipher is only secure if the keyword is long, random, and never reused. Short keywords reveal themselves through repeating patterns in the ciphertext (the basis of Kasiski's attack). One-time pads (where the key is as long as the message and never reused) are an unbreakable special case.
Easy: 3-letter keyword. Length and a thematic category are shown. Medium: 4-6 letter keyword. Length is shown. Hard: 6-8 letter keyword. No hints. ZZZ Lore: Drawn from your fictional company's canon.
Easy puzzles 60 pts, Medium 120, Hard 180, ZZZ Lore 150. Each hint reveals one keyword letter for −15 points (minimum 10 points).