The Internet is like an enormous public library. Here you can chat with friends, find help with your homework, listen to music, buy a new backpack.
At the same time, the Internet is a very secret place. Confidential information - credit-card numbers, private messages, banking information - is exchanged. How can something so public be so private, too?
The answer: cryptography (crip-TAH-gruh-fee), from the ancient Greek kryptos (hidden) and graphikos (writing).
When you think of codes, you think of government secrets and spies. But it was the invention of a reliable way to encode information in the late 1970s that led to the creation of the Internet.
But first, a little background on secret messages in general.
Why secrets are slippery
Suppose someone named Spy Guy wants to tell you a secret. He's going to send a hidden message to you in a letter. How will you read it if it's hidden? He's going to send you a "key" to the message in a separate letter.
A "key" gives you the directions you need to read a secret message. "Skip every other word" might be the key. Or "read it backward," or "A is E, B is F, C is G" and so on. Now you can read Spy Guy's message.
But what if Spy Guy's arch-enemy, Bad Guy, secretly steals both letters and copies them? Now he can read Spy Guy's message to you, too.
And even if he doesn't intercept the "key" letter, Bad Guy may have a friend who's a "cryptanalyst" - a codebreaker. The science of codebreaking, called "cryptanalysis," began with the Arabs around AD 500. The Arabs discovered a process that looks for word or letter patterns. Patterns help to reconstruct a key and unscramble the cipher. (Example: E is the most-used letter in English. If you find that most of the symbols in a secret message are, say, Z, chances are that Z means E.)
Secret or concealed messages have been around a long time. The Greek historian Herodotus (he lived from about 484 to 425 BC) mentions several clever ways the Spartans sent secret messages. One was to write a message on a wooden tablet, then smear the tablet with beeswax. The wax covered the message and made the tablet look like a plain wooden board. The receiver of the message could easily remove the wax and read the message.
"Rebel Rose" Greenhow was a spy for the Confederacy during the Civil War (1861-65). She moved in the elite social circles of the North. She would go to parties and eavesdrop, picking up military information from Union officers and others.
Then she would sew hidden pockets into her clothing or embroider secret messages onto her clothes. She would simply wear the coded message as she passed through Union lines to meet with Confederate agents.
Codes and ciphers are different, though today we generally use the word "code" for both. By definition, a code replaces entire words with other words or symbols. For example: "Eat" might be code for "attack." So a message saying "It's time to eat" means "It's time to attack."
A cipher replaces the individual letters in a message with different letters or symbols. Ciphers require a key to solve. If an enemy steals your key, or puzzles out your cipher using cryptanalysis, your secret is no longer safe. And so it was, for hundreds of years: Ciphers need keys, and keys make ciphers vulnerable to being read.
It began as a dream
In 1969, cryptographer Whitfield Diffie had a dream. He wanted to link computers together so they could communicate with one another. (Sound familiar? It's the Internet.) But first Mr. Diffie had a problem to solve: How could computers communicate secretly without having to transmit keys, which can be intercepted?
Fellow cryptographers Martin Hellman and Ralph Merkle joined Diffie in his search. And in 1976 they announced an amazing breakthrough in encryption (putting messages into code). The men published their discovery in a scientific paper, but they did not succeed in turning their discovery into a software program.
In April 1977, MIT researchers Ron Rivest, Adi Shamir, and Leonard Adelman put the Diffie-Hellman-Merkle idea into concrete form. They called the encryption process RSA, after the first initials of their last names. Their "public key encryption process," as it's called, is widely seen as the most significant encryption process in the history of modern cryptology.
There was no longer a concern about transmitting a key. That's because there were TWO keys - a public key, and a private one. (See "Every e-mail you send is written in code" at right.)
And so the Internet lived happily ever after, right?
For now it does. But it's likely that a new kind of computer will potentially make many encryption systems like RSA obsolete. Quantum computers will make today's most powerful computers look like abacuses.
Next: Quantum cryptography
Computers today speak in "binary" - a series of zeroes and ones. Quantum computers will use states of matter, called "quantum bits" or "qubits," to perform calculations - perhaps 1 billion calculations per second! A computer with that kind of power could probably crack the RSA code without too much trouble.
And then? Well, does anyone out there have any good ideas for an unbreakable code?
Take a virtual tour of the National Cryptologic Museum:
The CIA has a website for kids with history, quizzes, disguises to create, and codes to break.
For more on quantum computers, including IBM's latest progress toward one, go to:
www.research.ibm.com/resources/news/20000815_quantum.html The uncrackable code that wasn't
Nazi Germany's Enigma Code machine (left) looked like a typewriter. As each letter was typed on the keyboard, the encoded letter lit up on the top board. The secret message was then sent via radio in Morse Code. The Germans thought Enigma was unbreakable.
Using intercepted messages, captured Enigma machines, and a room-size computer named Colossus, British cryptographers cracked the code. But the fact that Enigma had been broken was kept secret until 1974.
(c) Copyright 2001. The Christian Science Monitor