Encryption
From Caesar's letter shift to the padlock of the internet
Summary
Encryption turns a message into a form that only someone with the key can read. Julius Caesar moved every letter three places along the alphabet; but ciphers like that are easily broken by looking at how often letters appear. The encryption we use on the internet today relies on two separate keys: one is public and locks the message, the other is secret and is the only one that unlocks it.
In depth
In the 9th century al-Kindi showed why shift ciphers are weak: in every language some letters appear far more often than others, and encryption does not change these frequencies, it only moves the letters along. So counting the letters in a long ciphertext gives the key away. The real revolution came in the 1970s: Diffie and Hellman, and then Rivest, Shamir and Adleman (RSA), found "public-key" encryption, which uses different keys to lock and to unlock. RSA's security rests on a simple fact: multiplying two large primes is easy, but splitting the product back into its primes is practically impossible for today's computers.
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