Vigenère Cipher: The Square Table

The weak spot of the Caesar cipher is that one shift covers the whole text. The Vigenère cipher, from the 16th century, uses a keyword instead: each letter of the key sets its own shift, and the key repeats under the message. For about three hundred years it was known as le chiffre indéchiffrable.

In the game you write a message and a keyword and encrypt the text through the Vigenère table: the row is the key letter, the column is the message letter, and the answer is where they cross. The ciphertext goes to Bob, who runs the table in reverse with the same key.

Works with the English alphabet (A–Z) and the Russian one.

Part of Ludus Lab: small browser games about cryptography and quantum key distribution, each with a short article. Theory and the other games: https://ludus-lab.com. Languages: English, Spanish, Chinese and Russian (buttons in the top-right corner).