This tutorial volume is based on a summer school on cryptology and data security held in Aarhus, Denmark, in July 1998. The ten revised lectures presented are devoted to core topics in modern cryptololgy. In accordance with the educational objectives of the school, elementary introductions are provided to central topics, various examples are given of the problems encountered, and this is supplemented with solutions, open problems, and reference to further reading. The resulting book is ideally suited as an up-to-date introductory text for students and IT professionals interested in modern cryptology.