CHAPTER 1 Introduction According t o legend , Juliu s Caesa r sen t encrypte d communication s t o Cicero because h e did no t trus t th e messenger s deliverin g his messages. Hi s scheme, whic h i s extremel y simple , i s base d o n elementar y mathematics . More tha n 2,05 0 year s later , compute r scientist s a t th e Massachusett s In - stitute fo r Technolog y (MIT ) develope d a n encryptio n metho d i n which th e enciphering detail s ar e mad e publi c an d ye t th e messag e remain s secure ! This schem e use s th e sam e mathematic s a s Caesar' s scheme , albei t i n a more sophisticate d way . A ciphe r i s a schem e fo r obscurin g a messag e i n orde r t o exchang e in - formation securely . Th e purpose o f using a cipher i s to kee p a message con - cealed fro m al l except th e intende d receiver . Privac y i s something we value, even whe n th e conten t o f a messag e i s no t particularl y significant . Tha t is wh y w e us e envelopes , instea d o f postcards , fo r al l bu t th e mos t trivia l of our writte n communications . I n thi s curren t "informatio n age, " w e send many differen t type s o f messages , includin g e-mail , ban k transactions , an d on-line shopping. Banks , stores, and librarie s all hold a tremendous amoun t of information abou t us . Th e nee d fo r privac y i s greater tha n ever . The us e o f ciphers i s probably a s old a s writing itself . On e o f the earli - est recorde d instance s occur s i n the boo k o f Jeremia h i n the Hebre w Bible . Throughout history , a variety o f ingenious techniques hav e been used to ob- scure or hid e messages . Thes e includ e writin g wit h invisibl e ink , employin g coding machines , an d hidin g th e rea l messag e insid e a large r one . A s a n example o f the latte r technique , conside r th e message : (1) A TRICKY TIGE R ALWAY S COME S KNOWINGLY. ^ ' A THREATENEDDEE R ALWAY S WALKS NEARBY. Can yo u fin d th e rea l message ? I f yo u writ e dow n th e firs t lette r o f eac h word i n (1) , you wil l find th e hidde n message : ATTACK AT DAWN. This schem e is , of course, no t particularl y secure . Dedicated codin g machines provide more security tha n th e schem e illus- trated i n (1). The y have the additional advantage of enabling the sender an d receiver t o cod e an d decod e message s quickly . Thoma s Jefferso n invente d one o f th e oldes t know n codin g machines i t i s o n displa y a t Monticell o i n Virginia. I n Worl d Wa r II , th e German s use d th e ENIGM A machine . A turning poin t i n th e wa r occurre d whe n th e allie s buil t thei r ow n cop y o f l http://dx.doi.org/10.1090/mawrld/025/01
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