Vernam Cipher: The One-Time Pad, Bit by Bit

The Vernam cipher, or one-time pad, is the only cipher proven to be unbreakable, under three conditions: the key is truly random, as long as the message, and used only once.

The game goes all the way down to bits. Choose an encoding (Baudot, ASCII 7 or 8 bit, UTF-8), turn the message into bits, generate a random key and combine the two with XOR, step by step or all at once. Bob applies XOR with the same key and gets the message back. Try a key shorter than the message and you will see why the conditions matter.

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).