The Vernam Cipher — The Only Unbreakable Cipher Ever Invented

Every cipher in this course so far can be broken, given enough ciphertext and enough patience. One cannot — not by a supercomputer, not by the entire internet, not by every machine that will ever exist. It is called the Vernam cipher, better known as the one-time pad, and it is the only cipher ever proven mathematically unbreakable. In this lesson we trace the cipher's history from Gilbert Vernam's 1917 teleprinter tape to Mauborgne's one-time pad discipline to Claude Shannon's 1949 proof of perfect secrecy. We walk through the mechanism — a single XOR between plaintext and key — and the four strict rules that turn that trivial operation into the gold standard of cryptography. We finish by showing intuitively why the cipher is unbreakable: for any given ciphertext, every possible plaintext of the same length is an equally valid decryption, so the ciphertext reveals nothing. Build your own one-time pad encryptor on the companion site, experiment with the four OTP rules, and see for yourself why perfect secrecy is a property of the information, not the attacker: https://kyri-cou.github.io/cryptography/ This lesson introduces the Vernam cipher and proves why, used correctly, it is unbreakable. In the next lesson we look at exactly how it breaks when those four rules are violated — including the famous VENONA program that cracked Soviet one-time pads during the Cold War.