
Title:
0146 Summary

Description:

[Evans] So we've reached the end of Unit 1.

Let me remind you what we've covered.

We covered symmetric cryptosystems and introduced the terminology

and definitions we need to talk about encryption.

In particular, we know what it means for a cryptosystem to be correct

that encryption and decryption are indeed inverses.

Defining security is a much trickier thing, and we talked about ways we could define security

for a symmetric cryptosystem.

We introduced the onetime pad, which is a very simple but important cryptosystem,

and it's all based on using the XOR operation.

As long as we have a perfectly random key that's as long as the message,

the onetime pad gives us perfect security.

We looked at a formal way to define what a perfect cipher means

and prove that the onetime pad has that property.

We also saw that in order to be perfect, a cipher has to be impractical

that the number of keys has to exceed the number of messages,

and that means that every cipher that's used in practice is potentially breakable.

We saw one very interesting example of that with how the Allies at Bletchley Park

were able to break the Lorenz cipher.

We talked about modern symmetric ciphers which take advantage of computing power,

following many of the same principles of the historical mechanical ciphers like Lorenz

but using modern computing power and new ideas about how to scramble data

to produce much more confusion and make things much more challenging for cryptanalysis

even when attackers have access to the huge amounts of computing power

available today.

I hope you enjoyed Unit 1 and have a good understanding

of some of the theory behind symmetric ciphers and how they're constructed.

In Unit 2 we'll look at how to actually use symmetric ciphers

to solve problems like sending a message securely between 2 parties

over an insecure channel like the Internet

and being able to use symmetric ciphers to play games online

and to do important things like managing passwords.

Hope to see everyone back for Unit 2.