0:00:00.000,0:00:02.000 [Evans] So we've reached the end of Unit 1. 0:00:02.000,0:00:05.000 Let me remind you what we've covered. 0:00:05.000,0:00:08.000 We covered symmetric cryptosystems and introduced the terminology 0:00:08.000,0:00:12.000 and definitions we need to talk about encryption. 0:00:12.000,0:00:16.000 In particular, we know what it means for a cryptosystem to be correct-- 0:00:16.000,0:00:20.000 that encryption and decryption are indeed inverses. 0:00:20.000,0:00:25.000 Defining security is a much trickier thing, and we talked about ways we could define security 0:00:25.000,0:00:27.000 for a symmetric cryptosystem. 0:00:27.000,0:00:32.000 We introduced the one-time pad, which is a very simple but important cryptosystem, 0:00:32.000,0:00:35.000 and it's all based on using the XOR operation. 0:00:35.000,0:00:40.000 As long as we have a perfectly random key that's as long as the message, 0:00:40.000,0:00:43.000 the one-time pad gives us perfect security. 0:00:43.000,0:00:47.000 We looked at a formal way to define what a perfect cipher means 0:00:47.000,0:00:50.000 and prove that the one-time pad has that property. 0:00:50.000,0:00:55.000 We also saw that in order to be perfect, a cipher has to be impractical-- 0:00:55.000,0:00:59.000 that the number of keys has to exceed the number of messages, 0:00:59.000,0:01:03.000 and that means that every cipher that's used in practice is potentially breakable. 0:01:03.000,0:01:07.000 We saw one very interesting example of that with how the Allies at Bletchley Park 0:01:07.000,0:01:10.000 were able to break the Lorenz cipher. 0:01:10.000,0:01:14.000 We talked about modern symmetric ciphers which take advantage of computing power, 0:01:14.000,0:01:18.000 following many of the same principles of the historical mechanical ciphers like Lorenz 0:01:18.000,0:01:23.000 but using modern computing power and new ideas about how to scramble data 0:01:23.000,0:01:28.000 to produce much more confusion and make things much more challenging for cryptanalysis 0:01:28.000,0:01:31.000 even when attackers have access to the huge amounts of computing power 0:01:31.000,0:01:33.000 available today. 0:01:33.000,0:01:36.000 I hope you enjoyed Unit 1 and have a good understanding 0:01:36.000,0:01:40.000 of some of the theory behind symmetric ciphers and how they're constructed. 0:01:40.000,0:01:43.000 In Unit 2 we'll look at how to actually use symmetric ciphers 0:01:43.000,0:01:46.000 to solve problems like sending a message securely between 2 parties 0:01:46.000,0:01:48.000 over an insecure channel like the Internet 0:01:48.000,0:01:51.000 and being able to use symmetric ciphers to play games online 0:01:51.000,0:01:54.000 and to do important things like managing passwords. 0:01:54.000,9:59:59.000 Hope to see everyone back for Unit 2.