[Evans] So rather than speculate on why that is, this brings us to the end of Unit 2.
Our main focus for Unit 2 was how to use symmetric encryption to solve problems.
For the first part we focused on this key,
the need to generate a random key, which is a very hard problem,
and we saw that we could use physical randomness
if we had enough available to do this,
but there's no way mechanically to produce a perfectly random key.
But we could use a pseudo-random number generator built using encryption
to take a small amount of random data and amplify that
to produce more pseudo-random data.
We saw how to use a symmetric cipher to take a small amount of random data
and produce a sequence of values that appear to be pseudo-random.
We also looked at the problem of how to encrypt a large file or a large message,
and that brought us to look at modes of operation for using symmetric ciphers.
We talked about the cipher block chaining mode,
the counter mode, and the cipher feedback mode,
all of which have different advantages and disadvantages.
We also looked at how to do fair coin tosses remotely,
and that led us to the need for cryptographic hash functions.
We saw how to use those to check user passwords
in a way that doesn't require us to keep any secrets on the server,
and we also saw how to use hash chains to make it
so those passwords never needed to be reused.
The big problem we haven't addressed yet is how to establish a shared key.
If we want to use a symmetric cipher to allow 2 parties to communicate,
they have to agree upon the shared key beforehand.
If you think about most ways we want to communicate today, that's very difficult.
If you visit a secure website, you don't have a shared secret with them to begin with,
but you want to start communicating securely with them despite not having a shared secret.
And for most uses of cryptography today this is a really big problem.
We can't assume that we have a shared secret with every website that we want to use.
We need some way to establish a shared secret without having one to begin with.
That's the main topic of Unit 3. I hope to see you there.
