"It was the amateurs of cryptology who created the species. The professionals, who almost certainly surpassed them in cryptanalytic expertise, concentrated on down-to-earth problems of the systems that were then in use but are now outdated. The amateurs, unfettered to those realities, soared into the empyrean of theory" (from David Kahn's The Code Breakers).
When one thinks of cryptography, one imagines spies and espionage, yet the art and practice of cryptography is going on about us everyday of our lives. Its art and practice is part of the fabric of society and we have all practiced this science in one way or another. In practice, cryptography is the science of transmitting information from one location to another, with the added requirement that this message cannot be understood by someone other than the sender and the receiver. Most people would accept this as the standard definition of this science. Yet, as you have read this, why is it that we have stated that this is going on about us everyday of our lives?
Every society has its methods of getting across information in secret. We are at a dinner party and we want to leave. We want to tell our spouse that it is time to leave without letting the hostess know of this. A slight movement of the eyes, a nudge, or a shuffle of our feet are all methods that we have used to get across this secret information. Another dinner guest seeing these movements might not understand the hidden message. That is the purpose of our secret cryptosystem, a system in which messages can be sent in secret and the intended receiver can then interpret correctly the message.
There are examples of cryptosystems noted throughout history. As early example is the following: The leader of one land needs to tell his friend, the leader of another land, that it is time to attack. Since he cannot simply tell his friend directly and risk alerting the enemy, he must pass on the message in secret. So, he shaves the head of a servant and write the message on the servant's bald head. Once the hair has grown back, he sends the servant to deliver the message to his friend. In order to retireve the message, the friend merely needs to shave the servant's head. So he attacks!
We have all heard the complaint from the older generation that they cannot understand the younger generation. Teenagers use language in a way that makes it difficult for the adults to understand. What teenagers are doing is creating a system of language that teenagers understand, yet their parents cannot! That is exactly the science of cryptography being applied. It is establishing a communication system whose purpose it is to allow confidential communication to occur among members of the network. This example alos displays another important component of a cryptosystem. A method of secret communication is designed to be used only for a fixed amount of time, then it is changed. Teenagers will be teenagers only for a little bit, so they really don't care if their system of communication is understood after a few years. In fact, it is possible that his phenomenon is one of the factors that is responsible for the slow but gradual change in language that is constantly occurring.
History is replete with escapades as to how unauthorized persons were able to get a hold of codebooks in order to be able to decode their enemy's secret transmissions. You have all seen movies in which the codebooks at an embassy had to be destroyed so that they would not fall into enemy hands. But it is sometimes possible to decipher a code without having the codebook available, thus the science of cryptanalysis. One famous example of this is the "Zimmermann telegram". During World War I, the British managed to break the German consular code, that is, the British, without knowing the codebooks that were used by the German embassies, were able to decode the messages between the embassies and the government in Berlin. The British, in 1917 intercepted a message from Arthur Zimmermann, the German Foreign minister, to the Mexican President. The decoded message proposed that Mexico declare war on the United States and that after Germany and her allies were victorious, Mexico could reclaim the territories of Texas, New Mexico and Arizona, that it had lost to the United States in 1846. The British turned this decoded message over to the Americans, who published it on March 1, 1917. The United States declared war on Germany the following month (Vélez, 1997).