Viruses dont harm, ignorance does. Is ignorance a defense? herm1t [...] I am convinced that computer viruses are not evil and that programmers have a right to create them, to possess them and to experiment with them ... truth seekers and wise men have been per- secuted by powerful idiots in every age ... Mark A. Ludwig Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers. Article 19 of Universal Declaration of Human Rights The purpose of this book is to propose a teaching approach to under- 1 stand what computer viruses really are and how they work. To do this, three aspects are covered ranging from theoretical fundamentals, to pr- tical applications and technical features; fully detailed, commented source 1 We will systematically use the plural form viruses instead of the litteral one virii. The latteris now an obsolete, though gramatically recommended, form. VIII Preface codes of viruses as well as inherent applications are proposed. So far, the applications-oriented aspects have hardly ever been addressed through the scarce existing literature devoted to computer viruses.