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 ciphertext underneath.

It's much more complicated, but

it scrambles data in a particular way, according to a particular recipe.

And it too takes in a key, and there are many,

many more keys than even that simple substitution cipher.

But it's fundamental to realize the difference between the recipe and the key.

And these are two critical features of any encryption process.

Now there are two very different types of encryption system, and

this is something that's worth flagging right now.

And if you go back and remember the analogy for

encryption is locking information away in a box,

it's actually helpful then to think about locks and keys for a moment.

Because there are two types of locking mechanisms we use

in the every day physical world.

There are locks where we need the same key to lock a box, and

we need that key to unlock the box, and we need the key on both parts of the process.

But there are also keys, like padlocks, for example,

where anyone can lock the box just by snapping the padlock shut, and

only the person who holds the key can unlock the box.

And if we think about unlocking as being decryption, what this tells us is in any

encryption mechanism, the decryption key will have to be a secret.

It has to be something held only by the intended recipient of some information.

But the locking key, the encryption key doesn't necessary have to be a secret.

And this defines two types of cryptography.

So in symmetric encryption, the encryption key and

the decryption key are the same thing.

And therefore, have to be secret.

But in public-key cryptography, rather like the padlock analogy,

the encryption key can be a piece of public information.

So anyone can encrypt something, and only the decryption key needs to be a secret.

We'll come back to the importance of that in a later lesson, but it's important at

this stage to realize these two very different types of cryptography exist.

So, in summary, encryption algorithms are recipes, they're ways of scrambling data.

And keys play a critical role,

because keys allow the data to be scrambled in different ways,

many different ways, more ways than there are stars in the universe, hopefully.

And there are two very different types of cryptography.

In symmetric cryptography, the encryption key and

decryption key are the same, and need to be held secret.

Whereas in public-key cryptography, the encryption key could be

something everybody knows, and only the decryption key needs to be held secret.

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