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:
Therequirementisandqarerelativelyprimeforthesamereasonasabove.
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)Ifandareinthesamerowinthekeysquarematrix,theciphertextlettersandarerespectivelyAndtherightletterof(thefirstlineisconsideredtobebelowthefifthline).
(2)Ifandareinthesamecolumn,thenandarerespectivelyandtotherightoftheletter(thefirstlineisregardedasthebottomofthefifthline).
(3)Ifandareindifferentrowsandcolumnsinthekeymatrix,theciphertextlettersandaretheothertwoverticesinarectanglecomposedofandasvertices,ofwhichand,andareonthesamelinerespectively.
(4)If=,insertaninvalidbetweenandLetters,suchasX.
(5)Iftheplaintextinformationhasanoddnumberofletters,aninvalidletterisappendedtotheendoftheplaintext.