Substitute password

Simplereplacementpassword

SimplereplacementpasswordistoreplaceeachletterintheelbowoftheplaintextalphabetwiththecorrespondingletterintheciphertextalphabetC.Thistypeofcipherincludesshiftciphers,multiplierciphers,affineciphers,polynomialsubstitutionciphers,andkeyphraseciphers.Beforeencryption,eachletterinthealphabetisgenerallynumberedaccordingtoitsposition.Forexample,0,1,2,...,25areusedtorepresentEnglishlettersa,b,c,...,z.

(1)Shiftthepassword.ShiftthelettersoftheplaintextalphabetMtotherightbykpositionsandmodulothelengthqoftheplaintextalphabettoobtaintheciphertextletters.Thisisthesimplesttypeofreplacementpassword.Itsencryptiontransformationcanbeexpressedas:

,thedecryptiontransformationis:,whereqisthelengthofthealphabetM,misthepositionoftheplaintextcharacterinthealphabetM,andcisthesecretThepositionoftheletterinthealphabetC.Theshiftpasswordisapasswordthatreplaces26Englishletters,whereq=26.ThiskindofcipherisalsocalledtheCaesarcipher,becausetheancientRomanCaesarusedthiscipherwhenk=3.Forexample,usingtheCaesarcipherencryption,theplaintextuniversitycanbeencryptedintociphertextqlyhuvlwb.

(2)Multiplierpassword.Multiplytheplaintextletterbythekeykandmoduloqtogettheciphertextletter.Theencryptionprocesscanbeexpressedas:

wherekandqarerelativelyprime,Thiswaythelettersinthealphabetwillproduceacomplexsetofresidues.Ifkandqarenotmutuallyprime,someplaintextletterswillbeencryptedintothesameciphertextletter,andnotallletterswillappearintheciphertextalphabet.

(3)Affinepassword.Theplaintextlettersarelinearlytransformedtoobtaintheciphertextletters,andtheencryptedformis:

Therequirementis

andqarerelativelyprimeforthesamereasonasabove.

SimplesubstitutionofpasswordsBecauseoftheuseofasinglemappingfromplaintexttociphertext,thefrequencydistributionofsinglelettersinplaintextisthesameasinciphertext,andcanbeeasilydecipheredbyusingletterfrequencyanalysis.

Multiplenamesorhomophonicsubstitutionpasswords

Inhomophonicsubstitution,theletteraofaplaintextalphabetcanbetransformedintoseveralciphertextletters,calledTheyarehomophones.Therefore,theformofmappingfromplaintexttociphertextis,whereAandCarethealphabetsofplaintextandciphertextrespectively.

Example:Assumingthatthekeyofahomophoneinsteadofapasswordisashorttext,thetextandthenumberofeachwordinitareasfollows:

(1)Canada'Slargelandmassand

(6)Scatteredpopulationmakeefficientcommunication

(11)anecessity.Extensiverailway,road

(16)andothertransportationsystems,as

(21)wellastelephone,telegraph,and

(26)cablenetworks,havehelpedto

(31)linkcommunitiesandhaveplayed

(36)avitalpartinthe

(41)country'sdevelopmentforfuture

Intheabovetable,thefirstletterofeachwordcorrespondstoanumber,suchastheletterCCorrespondstothenumbers1,10,26,32,4l;theletterMcorrespondstothenumbers4,8,etc.,whenencrypting,youcanuseanynumbercorrespondingtothelettertoreplacetheletter,forexample,iftheplaintextistheciphertextofILoveherforeverYes:

392173792891443171413371314

Multi-tablereplacementpassword

Mostmulti-tablereplacementpasswordsareperiodicInsteadofapassword,whentheperiodis1,itisasingletableinsteadofapassword.Therearemanytypesofmulti-tablereplacementpasswords,andonlytheVigenerepasswordandtheswimmingkeypasswordareintroducedhere.

IntheVigenerepassword,theuserkeyisafinitesequence,wecanextendktoaninfinitesequencethroughperiodicity(periodisd),Amongthem,Ki=K(imodd),1≤i≤,togettheworkkey.

IfΦandθareusedtorepresentciphertextandplaintextlettersrespectively,theconversionformulaofVigenerepasswordis:

Φ≡()(modn)

Thecryptosystemhasaparametern.Duringencryptionanddecryption,theEnglishlettersarealsomappedtonumbersfrom0to25,andthentheoperationsareperformed,andthelettersaretransformedaccordingtoagroupofletters.Theplaintextspace,theciphertextspace,andthekeyspaceareallsetsofEnglishletterstringsoflengthn.

Multi-letterreplacementpassword

Thecharacterblocksintheplaintextareencryptedingroups.Hereisatwo-letterreplacementpassword(Playfairpassword)usedintheFirstWorldWar.,Itskeyisafive-ordersquarematrixcomposedof25Englishletters(Jisremoved).

Eachpairofplaintextlettersandareencryptedaccordingtothefollowingfiverules:

(1)If

andareinthesamerowinthekeysquarematrix,theciphertextlettersandarerespectively

Andtherightletterof(thefirstlineisconsideredtobebelowthefifthline).

(2)Ifandareinthesamecolumn,thenandarerespectively

andtotherightoftheletter(thefirstlineisregardedasthebottomofthefifthline).

(3)Ifandareindifferentrowsandcolumnsinthekeymatrix,theciphertextlettersand

aretheothertwoverticesinarectanglecomposedofandasvertices,ofwhichand

,andareonthesamelinerespectively.

(4)If=,insertaninvalidbetweenandLetters,suchasX.

(5)Iftheplaintextinformationhasanoddnumberofletters,aninvalidletterisappendedtotheendoftheplaintext.