I wish to send a message to my friend Paul, but want to make sure his mother, who reads his post before he gets up, does not read it. I might choose to do this by writing the message on the inside of an envelope, and then posting an irrelevant but presentable letter and placing that in the envelope and sending it. Paul's mother would open the envelope and read my innocuous letter but not notice the writing in the envelope. Knowing our secret system, Paul would later take the envelope out of the waste paper bin and read my message.
The general 'hiding' of messages is known as stenography, and before the fifth century BC was the only method used to send secret information.
However, there are two problems with my envelope secretion system. One problem is that the system is by no means foolproof. If Paul's mother is particularly attentive, she may notice the hidden message, and once found, it is easy to read and my secrecy will be lost.
The other problem is that Paul needs to know that he has to look in the envelope. This means I have to tell him to look there, without his mother finding out. I certainly can't send a letter to him telling him the secret method, since his mother would intercept it and find out as well, so I would need a new way to pass on my hiding method, probably by meeting him at the pub on Friday night.
If Paul's mother should see the message, or even see him reading old envelopes, she may cotton on to the system and Paul and I would need a new way of communicating secretly. This would mean I'd have to meet him at the pub every time his mother worked out the system - a good excuse for a night out, you might think. But imagine I have many friends, all of whom have nosy mothers, and I have to meet them all at their local pub on Friday night to pass on my new method of hiding my messages to them. Now I haven't got enough time to stop at any of the pubs for long enough to have a pint, and the system is rapidly becoming impracticable.
The first problem, that of Paul's mother finding the message, is overcome by the introduction of cryptography, whereby the message is coded so that only Paul and I know how to read it. Then if his mother finds the secret message, she cannot read it anyway, and our secrecy is kept. In fact, cryptography is so much more powerful than stenography that there is little point in my hiding the message if she cannot read it, so I might as well write it boldly in the normal letter, in the knowledge that she will not be able to understand the content when she does read it.
However, this does not overcome the second problem. Before, I needed to meet Paul to tell him where I was going to hide the message, and now I still need to meet him to tell him how I am going to code the cryptic message, so that he knows how to decipher it.
The solution to this problem is far harder, and will be dealt with later on. For now it can be assumed that since I only have a few friends, I either have time to meet them individually, or have funds to pay someone I trust to meet them for me. This is a nasty assumption to have to make, so keep it in mind and we'll turn out a solution much later on.
