OverviewCryptography is a vital technology that underpins the security of information in computer networks. It is also a subject that has wide intrinsic appeal for people fascinated by "secrets" and "codes". This book presents a comprehensive introduction to the role that cryptography plays in providing information security for technologies such as the internet, mobile phones, payment cards and Bitcoin. The focus of the book is on what a user or practitioner needs to know about the cryptography that is deployed in everyday applications. The book thus considers the fundamental principles that underpin cryptography and examines how these influence the use of cryptography in practice. It also consider wider societal issues that arise concerning the use of cryptography following revelations by former NSA contractor Edward Snowden.