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Dialogue: 0,0:00:00.00,0:00:02.00,Default,,0000,0000,0000,,[Evans] So we've reached the end of Unit 1.
Dialogue: 0,0:00:02.00,0:00:05.00,Default,,0000,0000,0000,,Let me remind you what we've covered.
Dialogue: 0,0:00:05.00,0:00:08.00,Default,,0000,0000,0000,,We covered symmetric cryptosystems and introduced the terminology
Dialogue: 0,0:00:08.00,0:00:12.00,Default,,0000,0000,0000,,and definitions we need to talk about encryption.
Dialogue: 0,0:00:12.00,0:00:16.00,Default,,0000,0000,0000,,In particular, we know what it means for a cryptosystem to be correct--
Dialogue: 0,0:00:16.00,0:00:20.00,Default,,0000,0000,0000,,that encryption and decryption are indeed inverses.
Dialogue: 0,0:00:20.00,0:00:25.00,Default,,0000,0000,0000,,Defining security is a much trickier thing, and we talked about ways we could define security
Dialogue: 0,0:00:25.00,0:00:27.00,Default,,0000,0000,0000,,for a symmetric cryptosystem.
Dialogue: 0,0:00:27.00,0:00:32.00,Default,,0000,0000,0000,,We introduced the one-time pad, which is a very simple but important cryptosystem,
Dialogue: 0,0:00:32.00,0:00:35.00,Default,,0000,0000,0000,,and it's all based on using the XOR operation.
Dialogue: 0,0:00:35.00,0:00:40.00,Default,,0000,0000,0000,,As long as we have a perfectly random key that's as long as the message,
Dialogue: 0,0:00:40.00,0:00:43.00,Default,,0000,0000,0000,,the one-time pad gives us perfect security.
Dialogue: 0,0:00:43.00,0:00:47.00,Default,,0000,0000,0000,,We looked at a formal way to define what a perfect cipher means
Dialogue: 0,0:00:47.00,0:00:50.00,Default,,0000,0000,0000,,and prove that the one-time pad has that property.
Dialogue: 0,0:00:50.00,0:00:55.00,Default,,0000,0000,0000,,We also saw that in order to be perfect, a cipher has to be impractical--
Dialogue: 0,0:00:55.00,0:00:59.00,Default,,0000,0000,0000,,that the number of keys has to exceed the number of messages,
Dialogue: 0,0:00:59.00,0:01:03.00,Default,,0000,0000,0000,,and that means that every cipher that's used in practice is potentially breakable.
Dialogue: 0,0:01:03.00,0:01:07.00,Default,,0000,0000,0000,,We saw one very interesting example of that with how the Allies at Bletchley Park
Dialogue: 0,0:01:07.00,0:01:10.00,Default,,0000,0000,0000,,were able to break the Lorenz cipher.
Dialogue: 0,0:01:10.00,0:01:14.00,Default,,0000,0000,0000,,We talked about modern symmetric ciphers which take advantage of computing power,
Dialogue: 0,0:01:14.00,0:01:18.00,Default,,0000,0000,0000,,following many of the same principles of the historical mechanical ciphers like Lorenz
Dialogue: 0,0:01:18.00,0:01:23.00,Default,,0000,0000,0000,,but using modern computing power and new ideas about how to scramble data
Dialogue: 0,0:01:23.00,0:01:28.00,Default,,0000,0000,0000,,to produce much more confusion and make things much more challenging for cryptanalysis
Dialogue: 0,0:01:28.00,0:01:31.00,Default,,0000,0000,0000,,even when attackers have access to the huge amounts of computing power
Dialogue: 0,0:01:31.00,0:01:33.00,Default,,0000,0000,0000,,available today.
Dialogue: 0,0:01:33.00,0:01:36.00,Default,,0000,0000,0000,,I hope you enjoyed Unit 1 and have a good understanding
Dialogue: 0,0:01:36.00,0:01:40.00,Default,,0000,0000,0000,,of some of the theory behind symmetric ciphers and how they're constructed.
Dialogue: 0,0:01:40.00,0:01:43.00,Default,,0000,0000,0000,,In Unit 2 we'll look at how to actually use symmetric ciphers
Dialogue: 0,0:01:43.00,0:01:46.00,Default,,0000,0000,0000,,to solve problems like sending a message securely between 2 parties
Dialogue: 0,0:01:46.00,0:01:48.00,Default,,0000,0000,0000,,over an insecure channel like the Internet
Dialogue: 0,0:01:48.00,0:01:51.00,Default,,0000,0000,0000,,and being able to use symmetric ciphers to play games online
Dialogue: 0,0:01:51.00,0:01:54.00,Default,,0000,0000,0000,,and to do important things like managing passwords.
Dialogue: 0,0:01:54.00,9:59:59.99,Default,,0000,0000,0000,,Hope to see everyone back for Unit 2.