Welcome to Module 8 of the course! This module we move on to a different topic in cryptography: multiparty cryptography. The main difference compared to what we’ve seen so far is that…no more Eve! (Don’t be too sad, we’ll promise she’ll be back.) In multiparty cryptography the users, Alice and Bob (and sometimes their comrades Charlie, Daniela, and Fatima) engage in a simple task, such as flipping a coin or computing a function, but: now they no longer trust each other! We then try to find protocols that are secure from the point of view of the honest party, irrespective of possible malicious behavior from the other parties.

As usual we need to be very careful in making our notion of security precise. In 8.1 we’ll give you a first introduction and an intuitive security definition. In 8.2 and 8.3 we’ll discuss the two most important examples of tasks in two-party cryptography: oblivious transfer and bit commitment. In 8.4 we’ll see that, even though quantum information can sometimes lead to better protocol, perfect bit commitment (as well as oblivious transfer) are impossible in the classical as well as quantum world. Finally in 8.5 we’ll disc, ss one last example, where quantum information does provide an important advantage: coin tossing.

We hope you have a good time navigating through the zoo of multiparty cryptography, and come up with your own inventive protocols!