need a decryption algorithm to unscramble. But they'll also need a decryption key, and that's the thing that changesover time. And once again, this is probably best seen by an example. We're going to use an encryption algorithmthat's a lookup table; we're going to place letters in the top by letters underneath. But instead of having only one wayof doing this, we're going to make it the case that the letters underneath can be represented in any number ofdifferent ways.What is going to have to happen is that the sender and receiver is going to have to agree with how theencoding is done. The algorithm will still be a table, take the letter on top, and replace it by the letter underneath, butthe particular letter that's chosen will be the key, and that will be unknown by an attacker who observes thisciphertext. So, for example, if we take the following table where A is replaced by D, B by I, C by Q, etc.--in that case,the message, top secret, is now replaced by the ciphertext P R J W T Q U T P. But on the other hand, if we have acompletely different key and replace a by N, b by R, c by A, then on this occasion, the plaintext top secret isconverted into ciphertext X V J B K A D K X. And you can see that now there are a lot of different ways in which wecan replace the plaintext by ciphertext. And they all depend on different keys that the receiver has agreed with thesender before the encryption was used.Now, in general, we're going to need lots and lots of keys. And in fact, that way of encrypting we've justdiscussed is sometimes called the simple substitution cipher. And the question is, how many different ways couldwe have scrambled that message top secret? And the answer is 40,000 times more than the number of stars in ouruniverse, and that is a lot. So there is no way someone is going to get the correct key under this kind of system,even if they just try it randomly. Now that simple substitution cipher is fundamentally flawed in lots of different ways,which we'll not talk about.What is important to realize is that modern encryption algorithms, like the Advanced Encryption Standard,which is in many of the technologies we use every day, doesn't have these kinds of flaws.It, in itself, is a recipe, a way of scrambling data. Rather like just replace the plaintext letter by the ciphertextunderneath. It's much more complicated, but it scrambles data in a particular way, according to a particularrecipe. And it too takes in a key, and there are many, many more keys than even that simple substitution cipher. Butit's fundamental to realize the difference between the recipe and the key. And these are two critical features of anyencryption process. Now there are two very different types of encryption system, and this is something that's worthflagging right now.