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Peoplehaveusedsecretcodesforthousandsofyears.36Codebreakingneverlags(落后)farbehindcodemaking.Thescienceofcreatingandreadingcodedmessagesiscalledcryptography.Therearethreemaintypesofcryptography.37Forexample,thefirstlettersof“Myelephanteatstoomanyeels”Spelloutthehiddenmessage“Meetme.” 38Youmightrepresenteachletterwithanumber,Forexample,Let’snumberthelettersofthealphabet,inorder,from1to26.Ifwesubstituteanumberforeachletter,themessage“Meetme”wouldread“13520135.”Acodeusessymbolstoreplacewords,phrases,orsentences.Toreadthemessageofarealcode,youmusthaveacodebook.39Forexample,”bridge“mightstandfor“meet”and“out”mightstandfor“me.”Themessage“bridgeout”wouldactuallymean“Meetme.”40However,itisalsohardtokeepacodebooksecretforlong.Socodesmustbechangedfrequently.A.Itisveryhardtobreakacodewithoutthecodebook.B.Inanylanguage,somelettersareusedmorethanothers.C.Onlypeoplewhoknowthekeywordcanreadthemessage.D.Aslongastherehavebeencodes,peoplehavetriedtobreakthem.E.Youcanhideamessagebyhavingthefirstlettersofeachwordspellitout.F.Withacodebook,youmightwritedownwordsthatwouldstandforotherwords.G.Anotherwaytohideamessageistousesymbolstostandforspecificlettersofthealphabet.F.G.
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