People today are mcreasmgly relymg on pubhc computer networks to conduct bUSInessand take care of household needs. However, public networks may be insecurebecause data stored in networked computers or transmitted through networks canbe stolen, modified, or fabricated by malicious users. Thus, it is important to knowwhat securIty measures are aVaIlable and how to use them. Network securIty practreesare desIgned to prevent these potentIal problems. Network security, orIgmatedfrom meeting the needs of providing data confidentiality over public networks, hasgrown into a major academic discipline in both computer science and computerengineering, and also an important sector in the information industry.The goal of network securIty IS to gIve people the hberty of enJoymg computernetworks wIthout fear of compromlsmg theIr rIghts and mterests. Network secuntyaccomphshes thIs goal by provldmg confidentlahty, integrtty, non-repudIatIon,and availability of useful data that are transmitted in open networks or stored innetworked computers.Network securIty wIll remam an actIve research area for several reasons. FIrst,securIty measures that are effectIve today may no longer be effectIve tomorrowbecause of advancements and breakthroughs in computmg theory, algorIthms, andcomputer technologies. Second, after the known security problems are solved, othersecurity loopholes which were previously unknown may at some point be discoveredand exploited by attackers. Third, when new applications are developed or newtechnologIes are mvented, new securIty problems may also be created wIth them.Thus, network secunty IS meant to be a long lastmg scuffle between the offendersand the defenders.