Cryptography is split into two ways of changing the message systematically to confuse anyone who intercepts it: these are codes and ciphers. Many people believe, and use, the word code to mean the same thing as cipher, but technically they are different.
A code is a way of changing the message by replacing each word with another word that has a different meaning. For example, "Burn the City'' could become "Take the rubbish'' where the word "burn'' is represented by the codeword "take'', and similarly for "city'' and "rubbish''. Using codes requires a codebook, which contains all such codewords. Considering the large number of words in most languages, this is normally quite a large book, making the use of codes rather cumbersome (it is a bit like a french dictionary, giving the translation to and from the codeword). However, they can be used to encode key words in a message. Consider the message "Kill him as soon as possible''. With a simple change of a single word this becomes "Meet him as soon as possible'', which may pass through security detection without being noticed. So, although potentially hard to use, a simple code can be very effective, since even if the message is intercepted, they can be used so that the code reads as an innocent or unrelated topic.
Ciphers, on the other hand, convert the message by a rule, known only to the sender and recipient, which changes each individual letter (or sometimes groups of letters). Ciphers, are significantly easier to use than codes, since the users only have to remember a specific algorithm (a mathematical word for process) to encrypt the message, and not a whole dictionary of codewords. The major setback for ciphers compared to codes is that if someone finds a message that has been encyrpted using a cipher, the output is almost certainly going to be a random string of letters or symbols, and as such the interceptor will know straight away that someone wanted to hide this message.
The task of the cryptographer is to create a system which is easy to use, both in encryption and decryption, but remains secure against attempts to break it. For this reason, many ciphers have developed over the last 4,000 years to try to stop people from discovering what it is that their secret message says. In this website we focus our attention on ciphers, since they are more interesting and more diverse than the other forms of secret writing. We will be looking at many different ciphers, and will discuss how they work as well as some history behind their invention and use.
