What is cryptography?

Cryptography´s aim is to construct schemes or protocols that can still accomplish certain tasks even in the presence of an adversary. A basic task in cryptography is to enable users to communicate securely over an insecure channel in a way that guarantees their transmissions´ privacy and authenticity. Providing privacy and authenticity remains a central goal for cryptographic protocols, but the field has expanded to encompass many others, including e-voting, digital coins, and secure auctions. This paper explains what cryptography is about and how we can scientifically justify a cryptographic scheme´s security.