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International data encryption algorithm



Overview

TheInternationalDataEncryptionAlgorithm(IDEA,InternationalDataEncryptionAlgorithm)wasdevelopedbyresearchersXuejiaLaiandJamesL.MasseyatZurich’sETH,ownedbyaSwisscompanyAscomSystecpatent.IDEAisimplementedasaniterativeblockcipher,usinga128-bitkeyand8cycles.Bypayingroyalties(usuallyabout$6.00percopy),IDEAcanbewidelyusedallovertheworld.Thesefeesareapplicableinsomeregions,butnotinotherregions.IDEAisconsideredextremelysafe.Witha128-bitkey,thenumberoftestsrequiredinabruteforceattackwillincreasesignificantlycomparedwithDES,andevenweakkeytestingisallowed.Moreover,italsoshowsthatitisparticularlyresistanttoprofessionalformsofanalyticalattacks.

FeaturesofIDEAalgorithm

SimilartoDES,IDEAalgorithmisalsoadatablockencryptionalgorithm.Ithasdesignedaseriesofencryptionrounds,andeachroundofencryptionusesacompleteencryptionkey.Asubkeygeneratedinthekey.ThedifferencefromDESisthatitusessoftwaretoachievethesamespeedashardware.

SinceIDEAwasproposedanddevelopedoutsidetheUnitedStates,itavoidsmanyrestrictionsonencryptiontechnologyintheUnitedStates.Therefore,booksaboutIDEAalgorithmsandimplementationtechnologiescanbefreelypublishedandexchanged.ItcangreatlypromotethedevelopmentandperfectionofIDEA.Butbecausethealgorithmhasn'tappearedforalongtime,therearenotmanyattacksagainstit,andithasnotpassedthetestofalongtime.Therefore,itsadvantagesanddisadvantagescannotbejudgedyet.

IntroductiontoAlgorithms

IntroductiontoIDEAEncryptionAlgorithm

IDEA(InternationalDataEncryptionAlgorithm)belongstothecategoryofBlockCipherincryptography.IDEAusesakeywithalengthof128bitsandadatablocksizeof64bits.Theoreticallyspeaking,IDEAisa"strong"encryptionalgorithm,andtherehasbeennoeffectiveattackalgorithmonthisalgorithmsofar.

Asearlyas1990,XuejiaLaiandothersputforwardtheblockcipherproposalPES(ProposedEncryptionStandard)attheEuroCrypt’90annualmeeting.AttheEuroCrypt’91annualmeeting,XuejiaLaiandothersproposedarevisedversionofPES(ImprovedPES).CurrentlyIPEShasbeencommercializedandrenamedIDEA.IDEAhasbeenpatentedbytheSwisscompanyAscom,andthecompanymustapplyforalicensetousetheIDEAalgorithmforcommercialpurposes.

IDEAisaniterativealgorithmcomposedof8similarcircles(Round)andanoutputtransformation(OutputTransformation).EachcircleofIDEAconsistsofthreefunctions:modulo(2^16+1)multiplication,modulo2^16addition,andbitwiseXOR.

Beforeencryption,IDEAexpandsthe128-bitkeyto52encryptionkeysEK(EncryptionKey)throughKeyExpansion,andthencalculatesthedecryptionkeyDK(DecryptionKey)byEK).EKandDKaredividedinto8groupsofhalfkeys,eachgroupis6bytesinlength,thefirst8groupsofkeysareusedfor8-roundencryption,andthelasthalfgroupofkeys(4Byte)isusedforoutputtransformation.IDEA'sencryptionprocessanddecryptionprocessarethesame,butusedifferentkeys(EKforencryptionandDKfordecryption).

Theprocessofkeyexpansionisasfollows:

1.Usethe128-bitkeyasthefirsteightsub-keysofEK;

2.Shiftthefirst8bytetotheleftby25bittogetthenextsixsubkeys,andloopthisprocess7times;

3.Inthe7thcycle,takethefirst4bytesasthelast4bitsofEK;

4.Sofarthe52byteEKhasbeengenerated.

TheprocessofkeyexpansionisshowninTable1.Inordertoclearlyseetherelationshipofeach8Byte,each8ByteisdividedbythicklinesinTable1.

IDEAalgorithmisarelativelynewalgorithm,anditssecurityresearchisalsoongoing.SoonaftertheIDEAalgorithmwasannounced,somescholarspointedout:IDEA’skeyexpansionalgorithmisflawed,resultinginalargenumberofweakkeyclassesintheIDEAalgorithm,butthisweaknessisachievedbysimplymodifyingthekeyexpansionalgorithm(addinganexclusiveORoperator))Canbeovercome.Atthe1997EuroCrypt'97annualmeeting,JohnBorstandothersproposedtwoattackalgorithmsforIDEAwithreducedlaps:TruncateDiffrentialAttackon3.5-circleIDEAanddifferentialattackon3-circleIDEA.DiffrentialLinearAttack.Buttheauthoralsopointedoutatthesametimethatthesetwoattackalgorithmsareunlikelytoachieveasubstantialattackeffectontheentire8.5circleofIDEAalgorithm.Nonewattackalgorithmhasappearedyet.Itisgenerallybelievedthattheonlyeffectivewaytoattacktheentire8.5circleofIDEAalgorithmistosearchthe128-bitkeyspaceexhaustively.

Applicationexamples

Atpresent,IDEAhasalargenumberofapplicationexamplesintheproject.PGP(PrettyGoodPrivacy)usesIDEAasitspacketencryptionalgorithm;SecureSocketLayerSSL(SecureSocketLayer)alsoincludesIDEAinitsencryptionalgorithmlibrarySSLRef;Ascom,theowneroftheIDEAalgorithmpatent,hasalsolaunchedaseriesofsecurityproductsbasedontheIDEAalgorithm,including:IDEA-basedExchangesecurityplug-ins,IDEAencryptionchips,IDEAencryptionSoftwarepackages,etc.TheapplicationandresearchofIDEAalgorithmisbecomingmoreandmoremature.

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