Appendix A Cryptography Primer: Introduction to Cryptographic Principles and Algorithms Panayiotis Kotzanikolaou and Christos Douligeris A.1 INTRODUCTION As the term itself manifests, the original purpose of cryptography is message confidential- ity. 1 Modern cryptography, however, spans a wider range of topics. Formally speaking, “cryptography is the study of mathematical techniques related to aspects of information security such as confidentiality, data integrity, authentication and non-repudiation” [1]. This definition requires the definition of the basic security aspects of information security. Confidentiality is the property of protecting the content of information from all users other than the ones intended by the legal owner of the information. The nonintended users are generally called unauthorized users. Other terms such as privacy or secrecy have been used almost synonymously with confidentiality. Data integrity is the property of protecting information from alteration by unauthorized users. Authentication is divided into entity authentication and data authentication. Entity authentication is the property of ensuring the identity of an entity, which may be a human, machine, or other asset. Data origin authentication is the property of ensuring the source of the information. Finally, nonrepu- diation is the property of ensuring that entities that have committed to an action cannot deny that commitment at a latter time. Apart from the above basic security objectives, cryptography may assist in achieving several other security objectives of information systems, such as access control, anonym- ity, and timestamping. However, although cryptography is an essential element for network and systems security, it must be clear that cryptography by itself is not a synonym of security. In almost every information system and network, procedural measures, organi- zational techniques, and additional technical means are generally required in order to reach the desired security goals. 459 Network Security: Current Status and Future Directions, Edited by C. Douligeris and D. N. Serpanos Copyright © 2007 the Institute of Electrical and Electronics Engineers, Inc. 1 Cryptography comes from the Greek words kryptós, meaning hidden, and graphein, meaning writing. Thus, cryptography is hiding the actual message in a written script.