All right.

Then welcome to the cryptographic communication protocols lecture course.

I am Paul, Paul Rösler.

You can just call me Paul.

Yeah, this lecture will be about the security and cryptography that is important to formally understand what protocols are.

Before I write down my first notes, I want to mention that you can always ask questions, all the questions that you think are relevant.

I think it is always helpful for everybody else that you ask these questions,

because typically if you have the question, then you're not the only one who has that question.

There are no stupid questions, so please feel free.

We are very small course.

As you can see, maybe there will be further people coming a little bit late or something.

But I assume that we won't be much more people than than 10 people.

So we have all the time that you want to have for discussing things, understanding things.

And I think that's the most important part at the end,

that all of you understand what I want to tell you.

OK, maybe another second question.

All of you are master students, bachelor students, anyone?

Master students, master students, master students.

OK, perfect.

All of you took part in the introduction to modern cryptography.

No, OK, that's not a big problem.

I guess I will repeat the most important things.

I hope that all of you at some point had any contact with reduction based proofs,

because reductions are the things that we will use mostly for proving security.

But if not, we will have some examples at the beginning that hopefully will tell you a little bit of how it works.

But I won't go into that very deeply or formally anymore.

OK, so the course that I will be giving this semester is cryptographic communication protocols.

And the topic of today is split into three items.

I will first give a brief overview of the administration, things of the course contents and any other things that might be relevant.

We will then look at Diffie-Hellman key exchange, which is a super easy example to get in touch with cryptography again.

And the important things that we will have to look at when we want to consider more complex protocols like signals,

double ratchet and other things that happen in the messaging literature.

So Diffie-Hellman key exchange is super basic, but also very important.

And we will also look at L-gamma encryption, which is basically the Diffie-Hellman key exchange just with one multiplication.

And this offers us to build public key encryption.

And so at the very end of our day today, we will have formal definitions for public key encryption.

And if we have the time, also for key encapsulation mechanisms.

All right. So this is what we want to do today.

But as I said, we begin with an overview of administrative things.

And the most important one is the exam.

Since it seems as if we don't have much more than 15 people, it will, I guess, for now,

if it doesn't change within the next two and a half hours, like two hours, that there are far many more people here than the five of you,

then we will have an oral exam at the beginning of the semester break.

So I guess we will have the last week of the semester will just be a repetition and questions and answers lecture.

And then maybe two weeks after that, I will schedule the oral exam.

The oral exam is roughly 15, 20 minutes.

I don't know exactly, but I think 15 to 20 minutes, which consists of five minutes, a presentation from your side about your preferred topic on anything that happened during the course.

You can basically decide by yourself which topic you want to talk about.

The simpler the topic, the simpler probably my questions, the harder the topic, the harder my questions.

So basically, you can more or less decide in which range of of grades you want to be assessed from my side.