Data Communications Networking Devices - 4th Ed.pdf

7.4 SECURITY DEVICES ___________________________________________________________ 785thousands of years and have been used to protect a wide range of messages rangingfrom diplomatic and military communications from before the time of the RomanEmpire through commercial messages of industrial companies today.Manual techniquesIn spite of the fact that most manual coding techniques can be broken by trainedcryptanalysts, they do offer a measure of protection because of the time element:there may elapse a period of considerable length before the message is decoded byan unauthorized party. Thus, the information that company A will bid 2 milliondollars on a contract whose bid is to be opened on 1 February is worthless to acompetitor that decodes the message from the home of®ce to company A's ®eldagent after that date. In addition, manual coding schemes can also be used as abackup in the event of the failure of a coding device that you may have installed.Caesar CipherOne of the earliest coding techniques was the so-called Caesar Cipher, which wasprobably known and used long before Caesar was born. Using this technique, JuliusCaesar enciphered his dispatches by displacing each letter by a ®xed amount. If thedisplacement was two, then the message INVADE ENGLAND TONIGHTwould be transformed into KPXCFG GPINCPF VQPKIJV and sent bymessenger to the appropriate recipient. Upon receipt of the message, a reversetransformation would develop the plain text of the message. Although this schememay appear primitive, some encryption devices use electronic circuits or software toperform a continuous and alternating displacement of the plain text to develop anencrypted message which could frustrate the best cryptanalyst.Checkerboard techniqueA group of encryption techniques were developed based upon what is known as thebasic checkerboard. Here, the alphabet and numerals are written into a six by sixblock with coordinates for row and column used to specify the cells. This techniquecan involve the use of different indices as well as a rearrangement of cell data, asshown in Table 7.6. Again, with the advent of modern electronics and the use ofTable7.6Basic checkerboardBlock 1 Block 2A E I O U V G H I J K LA A B C D E F A Z Y X W V 0E G H I J K L B U T S R Q 1I M N O P Q R C P O N M L 2O S T U V W X D K J I H G 3U Y Z 0 1 2 3 E F E D C B 4V 4 5 6 7 8 9 F A 9 8 7 6 5Using the above blocks, the word CODE becomes: from Block1: AI II AO AU; from Block 2:EJ CH EI EH