ABSTRACT This chapter describes security threats that systems face when they are connected to the Internet. We discuss their security requirements, potential security threats and di erent mech-anisms to combat these. In addition, the text presents the two most popular protocols (SSL and its successor TLS) to secure data transmitted over the Internet. Finally, we describe well-known applications such as Secure Shell (ssh) and Secure File Transfer Protocol (sftp) that provide a reasonable level...