The Real World

1.

There is no secure channel over which we can send keys. Furthermore we can assume that since we know, and the person we are communicating with knows, the algorithm we are using then so does the rest of the world. There can be no obfuscation!

2.

We would like to be able to authenticate the sender of the message, to be sure it is who we think it is.

3.

We can assume that anyone who wishes to crack our encrypted message has access to a powerful computer that can be used to crack small keys and to search for any patterns. Desktop computers are rapidly approaching the point where they can complete 1 billion calculations a second on floating point numbers, and this is before we even start to consider super-computers.

It is easy to see how 1 applies to sending messages (credit card numbers maybe) over the internet, but the same principles could be applied if the only method of communication is via radiowaves - or even the telephone.

The transmission of Credit Card numbers can again be used as an example for why we might wish for 2.

equirement 3 really considers the problems in achieving the first 2. It is, however, important since we are reminded of the work that went on at Bletchley Park during World War II. Here early computers were used, in combination with `traditional' code-breaking techniques to crack the Lorenz Ciper used by the Nazi Enigma Machines. Each Enigma machine had 3 wheels which could each be put in any starting position and came with tables showing which starting positions should be used on which dates (i.e. the starting positions became the key). However the use of early computers meant that this key-space (the number of possible keys) was not big enough to prevent the code being cracked. The Nazi U Boats (submarines) had 4 wheel machines, increasing the number of keys and making their transmissions much harder to crack. Unfortunately for them, sloppy use combined with the Allies correctly guessing the content of some messages (such as weather reports) allowed even these transmissions to be cracked.