Welcome to this MOOC which is entitled: code-based cryptography. This MOOC is divided in five weeks. The first week, we will talk about error-correcting codes and cryptography, this is an introduction week.Then, we will introduce the McEliece cryptosystem, and the security proof for the McEliece cryptosystem. We will talk about generic attack, message attack, in other words. This course will be given by Nicolas Sendrier. Then, we'll give structural attacks or key attacks, and at the end, we will talk about othercryptographic constructions relying on coding theory. This last week will be given by Matthieu Finiasz. Today is the first sequence of this first week where we will give a brief introduction to cryptography and we will explain why it is interesting code-based cryptography. Cryptography is the science of keeping secrets secret. T