The use of codes and ciphers to protect secrets, began thousands of years ago.
Then came the methods of encryption that use pen and paper, or perhaps simple mechanical aids.
The development of cryptography has been paralleled by the development of cryptanalysis, i.e. the breaking of codes.
Ancient Spartan technology
In around 600 BC, the ancient Spartans use a device called a scytale to send secret messages during battle. This device consists of a leather strap wrapped around a wooden rod. The letters on the leather strip are meaningless when it's unwrapped, and only if the recipient has the correctly sized rod does the message make sense.
Roman encryption and ciphers
Julius Caesar invents a substitution cipher that shifts characters by three places: A becomes D, B becomes E, and so on.
Giovan Battista Bellaso envisions the first cipher to use a proper encryption key - an agreed-upon keyword that the recipient needs to know if he or she wants to decode the message.
Charles Wheatstone invents the Playfair Cipher, which encrypts pairs of letters instead of single ones and is, therefore, harder to crack.
Hebern rotor machine
An American, Edward Hebern, invents the electro-mechanical machine in which the key is embedded in a rotating disc. It's the first example of a rotor machine. It encodes a substitution table that is changed every time a new character is typed.
German engineer Arthur Scherbius invents the Enigma machine (pictured) for commercial use. Rather than the one rotor used by Hebern's device, it uses several. Recognizing its genius, the German military begins to use it to send coded transmissions.
Polish cryptographer Marian Rejewski discovers how Enigma works. In 1939, Poland shares this information with the French and British intelligence services, allowing cryptographers like Alan Turing to figure out how to crack the key, which changes daily. It proves crucial to the Allies' World War II victory.
Claude E. Shannon of Bell Labs publishes an article called "A mathematical theory of cryptography". It's the starting point of modern cryptography.
IBM forms a 'crypto group', which designs a block cypher to protect the company's customers' data. In 1973, the US adopts it as a national standard - the Data Encryption Standard, or DES. It remains in use until it's cracked in 1997.
DES is replaced by the Advanced Encryption Standard, or AES, which is found through a competition open to the public. Today, AES is available royalty-free worldwide and is approved for use in classified US government information.
As more and more services move to the cloud, encrypting data in transit is crucial, and cryptographers are continually developing and refining solutions to this challenge.