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1、Chapter2PrinciplesofModernSteganographyandSteganalysisThefirstworkondigitalsteganographywaspublishedin1983bycryptogra-pherGustavusSimmons[217],whoformulatedtheproblemofsteganographiccommunicationinanillustrativeexamplethatisnowknownastheprisoners’problem1.Twoprisonerswanttocookupanesca
2、peplantogether.Theymaycommunicatewitheachother,butalltheircommunicationismonitoredbyawarden.Assoonasthewardengetstoknowaboutanescapeplan,oranykindofscrambledcommunicationinwhichhesuspectsone,hewouldputthemintosolitaryconfinement.Therefore,theinmatesmustfindsomewayofhidingtheirsecretmess
3、agesininconspicuouscovertext.2.1DigitalSteganographyandSteganalysisAlthoughthegeneralmodelforsteganographyisdefinedforarbitrarycom-municationchannels,onlythosewherethecovermediaconsistofmultimediaobjects,suchasimage,videooraudiofiles,areofpracticalrelevance.2Thisissoforthreereasons:first
4、,thecoverobjectmustbelargecomparedtothesizeofthesecretmessage.Eventhebest-knownembeddingmethodsdonotallowustoembedmorethan1%ofthecoversizesecurely(cf.[87,91]inconjunctionwithTableA.2inAppendixA).Second,indeterminacy3inthecoverisnecessarytoachievesteganographicsecurity.Largeobjectswith
5、-outindeterminacy,e.g.,themathematicalconstantπatveryhighprecision,areunsuitablecoverssincethewardenwouldbeabletoverifytheirregular1Theprisoners’problemshouldnotbeconfusedwiththebetter-knownprisoners’dilemma,afundamentalconceptingametheory.2Artificialchannelsand‘exotic’coversarebrieflyd
6、iscussedinSects.2.6.1and2.6.5,respectively.3Unlessotherwisestated,indeterminacyisusedwithrespecttotheuninvolvedobserver(warden)throughoutthisbook.Theoutputofindeterministicfunctionsmaybedetermin-isticforthosewhoknowa(secret)internalstate.11122PrinciplesofModernSteganographyandSteganal
7、ysisstructureanddiscovertracesofembedding.Third,transmittingdatathatcontainsindeterminacymustbeplausible.Imageandaudiofilesaresovitalnowadaysincommunicationenvironmentsthatsendingsuchdataisincon-spicuous.Asinmoderncryptography,itiscommontoassumethatKerckhoffs’prin-ciple[135]isobeyedindi
8、gital
