A Brief History of Cryptography

by William O'Grady for Blog
A Brief History of Cryptography

Nowadays, most of us take cryptography for granted. In every email we send, online purchase we make, every WhatsApp message we send, we are benefiting from centuries of cryptography. There has always been the need to send messages and to only have certain people read them.

An early (and well-known) example of cryptography is the Caesar Cipher, named after Roman General and Dictator, Julius Caesar. When sending private correspondence, Caesar would shift each letter of the alphabet down a certain number of letters. This number of letters (the key) would need to be known by the recipient in order to decipher the message. They would simply reverse the shift to decipher the message.

caesarcipher

The next logical step in creating a cipher was to randomly assign each letter to another. The key in this case, would be each letter and its random match. This is a much more powerful cipher due to the huge number of possible keys. In response to substitution ciphers, Frequency Analysis was used. In the English language, some letters are used more than others. Think of Scrabble – “E” is much more common than “Z”. If you study the frequency of each letter in English, something like a digital fingerprint will appear as seen below: 

frequencyanalysis

Frequency Analysis

Note that these are only averages. In the secret message the letter E may not be the most common. However, the larger the piece of text, the more it will resemble the above frequencies. It was now possible to crack the Substitution Cipher by deciphering one letter at a time. 

There has always been evolution with cryptography. A new cipher is constantly under attack from code breakers. As a result, cryptographers have looked for more complex ciphers throughout history. The Vigenère Cipher was a considered a breakthrough for code makers.

“It has the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar ciphers. At different points in the encryption process, the cipher uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword.”

No longer would the most common letter be the letter E. The letter E would have different ciphers at different points in the message. It wasn’t until 1854, over two hundred years later, that the Vigenère Cipher was finally cracked by the British cryptographer Charles Babbage. Babbage employed a mix of cryptographic genius, intuition and sheer cunning to break the Vigenère Cipher.  Below, is a Vigenère Square which is used to encipher a message.

vigenere

With increased computational power in the 20th Century, cryptography has taken another leap forward. Famously, the Enigma machine (with a possible 150,738,274,937,250 keys  ) used by Germany in World War 2 required the use of the “Bomb” computer developed with the help of Alan Turing.

Modern Cryptography

In the age of online communication, there has been a need for greater security when transmitting messages. The issue that has always existed in the history of secret messages is protecting the key. The recipient of a message needs to have the key in order to decipher it. How then can you safely get the key to the recipient without it falling into the wrong hands?

One solution was to use a mathematical operation that is easy to do in one direction, but not in the other. For example, you can multiply 453,658 by 789,624 and in no time have 358,219,244,592. However, if I gave you the number 358,219,244,592 and asked you to find the two specific numbers I multiplied to get it, it would be a much tougher task. Image such a problem on an exponentially larger scale and you can start to see the difficulty. The issue now became finding a key, I.e. finding an operation that was easy to reverse only if you have a mathematical key.

In 1974, GCHQ’s Clifford Cocks was able to solve this problem by writing a one directional formula. It allowed messages to be sent over the internet where anyone could read the encrypted text. However, only the recipient would be able to decipher it using another of his formulae.

Nowadays, we can send messages that cannot be cracked by the best secret services in the world. For the first time in history, we all have access to unbreakable codes.

 

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