## ← Introduction - Applied Cryptography

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Showing Revision 2 created 05/24/2016 by Udacity Robot.

1. Welcome to Unit 3.
2. In Units 1 and 2, we learned about symmetric ciphers and how to use them.
3. Symmetric ciphers assume that both parties have the same key.
4. The important property that makes them symmetric
5. is that the same key is used for encrypting and decrypting.
6. In some scenarios, this is okay.
7. When we talked about using symmetric ciphers to encrypt a file,
8. we could assume the same party is encrypting and decrypting the file,
9. so they can create and keep the key locally.
10. But when we talk about using symmetric ciphers for most applications
11. that involve more than 1 party, this is a big problem.
12. If Alice and Bob want to talk together,
13. they have to first agree on the secret key.
14. There has to be a way for Alice and Bob to
15. communicate that key without exposing it.
16. They can't send it over the insecure channel.
17. If they could send a secret key over an insecure channel, well,
18. they wouldn't need encryption at all.
19. In the early days, this was done by having a secret code book
20. that was physically distributed to the end points that were communicating.
21. With Colossus, this was the code book that had the keys for each day,
22. and that was physically distributed to the capitols where the Lorenz machines were.
23. With Enigma, this was physically distributed to all the ships,
24. and captains were under very strict orders to destroy those code books
25. any time their ship was in danger of being captured.
26. This was the most important thing to destroy on the ship
27. to make sure the enemy didn't capture the code book.
28. For most applications, especially any civil application in the day of internet,
29. distributing a physical code book or having 2 parties meet
30. in a secure place to exchange a key is not very practical.
31. What we want is some way to establish a secure key
32. or some way to enable 2 parties to communicate
33. that doesn't require that, so that's the main topic of this unit.
34. We're going to look at different ways that we could enable this kind of communication
35. without starting from advance where Alice and Bob have a shared key.