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DESalgorithmentryparameters

TherearethreeentryparametersofDESalgorithm:Key,Data,Mode.Keyis7bytesand56bitsintotal,whichistheworkingkeyoftheDESalgorithm;Datais8bytesand64bits,whichisthedatatobeencryptedordecrypted;ModeistheworkingmodeofDES,andtherearetwotypes:encryptionOrdecrypt.

ThebasicprinciplesofDES

ThetwoprinciplesofblockcipherdesignareusedinthedesignofDES:confusionanddiffusion,thepurposeofwhichistocounterthestatisticsofthecryptographicsystembytheadversaryanalyze.Confusionistomaketherelationshipbetweenthestatisticalpropertiesoftheciphertextandthevalueofthekeyascomplicatedaspossible,sothatthedependencebetweenthekey,theplaintextandtheciphertextisunavailabletothecryptanalyst.Thefunctionofdiffusionistoapplytheinfluenceofeachplaintexttomoreoutputciphertextbitsasquicklyaspossible,soastoeliminatethestatisticalstructureoftheplaintextinalargenumberofciphertexts,andmaketheinfluenceofeachkeyasquicklyaspossibleThegroundisextendedtomoreciphertextbitstopreventdecipheringthekeypiecebypiece.

Algorithmsteps

TheDESalgorithmturnsa64-bitplaintextinputblockintoa64-bitciphertextoutputblock,andthekeyitusesisalso64bits(56bitsareactuallyused)The8,16,24,32,40,48,56,and64bitsarecheckdigits,sothateachkeyhasanoddnumberof1).Thealgorithmismainlydividedintotwosteps:

1)Initialreplacement

Itsfunctionistorecombinetheinput64-bitdatablockbitbybit,anddividetheoutputintotwopartsL0andR0,eachpartis32bitslong,anditsreplacementruleistoinputThe58thplacewaschangedtothefirstplace,the50thplacewaschangedtothesecondplace...andsoon,thelastplaceistheoriginal7thplace.L0andR0arethetwopartsafterthetranspositionoutput,L0istheleft32bitsoftheoutput,R0istheright32bits,forexample:settheinputvaluebeforethereplacementtoD1D2D3...D64,thentheresultaftertheinitialreplacementis:L0=D58D50……D8;R0=D57D49……D7.

Thereplacementrulesareshowninthefollowingtable:

58,50,42,34,26,18,10,2,

60,52,44,36,28,20,12,4,

62,54,46,38,30,22,14,6,

64,56,48,40,32,24,16,8,

57,49,41,33,25,17,9,1,

59,51,43,35,27,19,11,3,

61,53,45,37,29,21,13,5,

63,55,47,39,31,23,15,7,

2)Inversepermutation

After16iterations,L16andR16areobtained,whichareusedasinputtoperforminversepermutation.Inversepermutationisjusttheinverseoperationofinitialpermutation,thusGettheciphertextoutput.

Thisalgorithmisarepresentativeofthesymmetricencryptionalgorithmsystemandiswidelyusedincomputernetworksystems.

DESand3DES

3DES(ieTripleDES)isanencryptionalgorithmtransitioningfromDEStoAES.Itusesthree56-bitkeystoencryptdatathreetimes.ItisasafervariantofDES.ItusesDESasthebasicmoduleanddesignsablockencryptionalgorithmthroughacombinedblockmethod.ComparedwiththeoriginalDES,3DESismoresecure.

ThismethodusestwokeystoexecutetheDESalgorithmthreetimes.Theencryptionprocessisencryption-decryption-encryption,andthedecryptionprocessisdecryption-encryption-decryption.

The3DESencryptionprocessis:C=Ek3(Dk2(Ek1(P)))

The3DESdecryptionprocessis:P=Dk1(EK2(Dk3(C)))

Theadvantagesofusingtwokeysfortripleencryptionareasfollows:

①Thetwokeyscombinedhaveaneffectivekeylengthof112bit,whichcanmeettheneedsofcommercialapplications.Ifthetotallengthis168bitThethreekeyswillgenerateunnecessaryoverhead.

②Encryption-decryption-encryptionisusedforencryptioninsteadofencryption-encryption-encryption,whicheffectivelyrealizesthebackwardcompatibilitywiththeexistingDESsystem.BecausewhenK1=K2,theeffectoftripleDESisthesameastheoriginalDES,whichhelpstograduallypromotetripleDES.

③TripleDEShassufficientsecurity,andthereisnoreportonthebreachof3DES.