The goal of cryptography is to make it possible for two people to exchange a message in such a way that other people cannot understand the message. There is no end to the number of ways this can be done, but here we will be concerned with methods of altering the text in such a way that the recipient can undo the alteration and discover the original text.
The original text is usually called ``cleartext'' and the encoded or altered text is called ``ciphertext''. The conversion from cleartext to ciphertext is called ``encoding'' or ``enciphering'', and the opposite operation is called ``decoding'' or ``deciphering''. If you are trying to read a secret message that was not intended for you and you initially don't know the encoding method, it is called ``cracking'' the code.
In general, the more ciphertext you have, the easier it is to crack the code. So generally it is a good idea to change the coding mechanism regularly. For example, if a coding scheme has a keyword (like the Vigenère cipher described below), if a different keyword is used every day, there may never be enough ciphertext to decode the message. But if you change the encoding every day, you need to have some method of getting the new keyword to the intended recipient in a secure way. The easiest way to crack a code is to steal the codebook!
There is a way around this that we'll discuss in the section on public key cryptography.