Chapter 5 Cryptography

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1、CHAPTER5CryptographyZHQMZMGMZMFMGJuliusCaesarKXJEYUREBEZWEHEWRYTUHEYFSKREHEGOYFIWTTTUOLKSYCAJPOBOTEIZONTXBYBWTGONEYCUZWRGDSONSXBOUYWRHEBAAHYUSEDQJohnFKennedy5.1IntroductionCryptographyiswheresecurityengineeringmeetsmathematics.Itprovidesuswiththetoolsthatunderliemostmodernsecurityprotocol

2、s.Itisprobablythekeyenablingtechnologyforprotectingdistributedsystems,yetitissurprisinglyhardtodoright.AswevealreadyseeninChapter3,Protocols,cryptographyhasoftenbeenusedtoprotectthewrongthings,orusedtoprotecttheminthewrongway.Wellseeplentymoreexampleswhenwestartlookingindetailatrealapplic

3、ations.Unfortunately,thecomputersecurityandcryptologycommunitieshavedriftedapartoverthelast25years.Securitypeopledontalwaysunderstandtheavailablecryptotools,andcryptopeopledontalwaysunderstandthereal-worldproblems.Thereareanumberofreasonsforthis,suchasdifferentprofessionalbackgrounds(comp

4、uterscienceversusmathematics)anddiffer-entresearchfunding(governmentshavetriedtopromotecomputersecurityresearchwhilesuppressingcryptography).Itremindsmeofastorytoldbyamedicalfriend.Whileshewasyoung,sheworkedforafewyearsinacountrywhere,foreconomicreasons,theydshortenedtheirmedicaldegreesan

5、dconcentratedonproducingspecialistsasquicklyaspossible.Oneday,129130Chapter5■Cryptographyapatientwhodhadbothkidneysremovedandwasawaitingatransplantneededherdialysisshuntredone.Thesurgeonsentthepatientbackfromthetheateronthegroundsthattherewasnourinalysisonfile.Itjustdidntoccurtohimthatapat

6、ientwithnokidneyscouldntproduceanyurine.Justasadoctorneedstounderstandphysiologyaswellassurgery,soasecurityengineerneedstobefamiliarwithcryptologyaswellascomputersecurity(andmuchelse).Thischapterisaimedatpeoplewithoutanytrainingincryptology;cryptologistswillfindlittleinitthattheydontalread

7、yknow.AsIonlyhaveafewdozenpages,andaproperexpositionofmoderncryp-tographywouldrunintothousands,Iwontgointomuchofthemathematics(therearelotsofbooksthatdothat;seetheendofthechapterforfurtherreading).Illjustexplainthebasicintuitionsandconstructionsthatseemtocausethemos