Data Communications Networking Devices - 4th Ed.pdf

784 ___________________________________________________________ SPECIALIZED DEVICESFigure7.35 Intervention impact. With intervention software,the probability of successfulentry by random password selection becomes extremely smallWhile the preceding discussion was primarily concerned with sign-on access,similar problems and solutions can be effected with respect to the passwordsemployed to protect data ®les.Transmission securityAlthough a number of methods have been developed to promote access anddatabase security further; unless the transmission medium utilized is secure, youcan become vulnerable to having data transmission compromised by such means asline tapping or line monitoring. In addition, if transmission is over packet ormessage switching systems, or by courier or telegram, the message may be read byunauthorized personnel or obtained through active penetration by insiders as wellas outsiders. In certain US Department of Defense installations, transmission isover secure transmission lines which makes access security their major concern.This is due to the fact that, although all transmission lines on the installation aresecure, personnel with different security level clearances have access to the samecomputer and a method of differentiating who obtains what computer resources isprimarily determined by a terminal's location and the identi®cation code of theuser.For the situation where data transmission occurs over public, non-securefacilities, a method of making the transmitted data unintelligible to unauthorizedparties becomes as important as having a good access security procedure. Fortunately,numerous techniques are available for users to make transmitted dataunintelligible to unauthorised parties; the oldest and most widely used method isthe various types of manual coding processes where, through the use of code booksand pads, the original text is encoded before transmission and then decoded by therecipient of the message. The foundations for the various coding schemes go back