Hyperlinked definitions and discussions of many cryptographic, mathematics, logic, statistics, and electronics terms used in cipher construction and analysis. http://www.ciphersbyritter.com/GLOSSARY.HTM
A comprehensive security guide aimed at administrators and users, available online in Acrobat PDF format, by Kurt Seifried. https://www.seifried.org/lasg/
A small collection of Linux security white papers and other information resources. The papers cover secure shell (ssh), host security, and firewalls. http://www.ists.dartmouth.edu/IRIA/knowledge_base/
General overview of security issues that face the administrator of Linux systems. It covers general security philosophy and a number of specific examples of how to better secure your Linux system from intruders. http://scrye.com/~kevin/lsh/t1.html
Brief introduction to the IP networking services, how to configure them, and how to set up a relatively secure Linux workstation. Published in ACM Crossroads, the ACM student magazine. http://www.acm.org/crossroads/xrds6-1/linuxsec.html