From Caesar's cipher to public-key cryptography — how the art of keeping secrets became a science, and why it now underpins the entire internet.
For most of history, cryptography was a cat-and-mouse game between codemakers and codebreakers, played by generals and spies. In the last fifty years it became something else: a rigorous science, and then invisible infrastructure that every one of us relies on thousands of times a day. This short book traces that arc.
Two thousand years apart: shifting letters by hand, and the two-key mathematics that now secures the internet.
The oldest idea in cryptography is substitution: replace each letter with another. Julius Caesar shifted every letter three places down the alphabet — the Caesar cipher. It is trivial to break today, and it hides a lesson that took centuries to fully learn: the secrecy must live in the key (here, the shift amount), not in the method. A cipher whose only defence is that the enemy doesn't know how it works will not survive contact with a clever adversary.
Substitution ciphers ruled for a thousand years until Arab scholars developed frequency analysis — noticing that "e" is common in English, so the most common symbol in the ciphertext is probably "e". Suddenly simple substitution was broken, and the arms race began.
By the 20th century, cryptography was mechanised. The German Enigma machine of World War II scrambled each letter through rotating wheels, producing a cipher that changed with every keystroke and offered an astronomical number of settings. It seemed unbreakable — but a combination of mathematical insight (led by Marian Rejewski's Polish team, then Alan Turing at Bletchley Park), captured materials, and operator mistakes cracked it. The effort helped shorten the war and, along the way, invented much of modern computing.
The deeper lesson of Enigma echoes Caesar's: a system is only as strong as its weakest link — and humans, procedures and reused settings are usually weaker than the maths.
Every cipher so far shared one flaw: sender and receiver needed the same secret key, which somehow had to be exchanged in advance over a secure channel. For two strangers on the open internet, that is impossible.
In 1976, Whitfield Diffie and Martin Hellman published an idea that sounded absurd — a way for two parties to agree on a shared secret in public, where an eavesdropper hears everything and still cannot compute the secret. Soon after, RSA gave each person a public key to lock messages and a private key to unlock them. Cryptography's secrecy no longer depended on a pre-shared secret, but on mathematical problems that are easy to compute forwards and effectively impossible to reverse.
That single shift turned cryptography from a tool of spies into the foundation of e-commerce, messaging and the web. Every padlock in your browser is public-key cryptography at work — the direct descendant of Diffie and Hellman's impossible- sounding idea. (Educatifu's Cyber Security track lets you play with both the hashing and the key exchange behind it.)
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