Whatisatranspositionpassword?
Beforetheadventofcomputers,cryptographyconsistedofcharacter-basedpasswordalgorithms.Differentcryptographicalgorithmsaresubstitutionsbetweencharactersortranspositionsbetweeneachother.Agoodpasswordcombinesthesetwomethodsandperformsmultipleoperationseachtime.Thesecurityofthesealgorithmsisbasedontheconfidentialityofthealgorithm.Oncethealgorithmisleaked,itiseasytobedeciphered.Today,theyareallrelativelysimplepasswords.Althoughtheencryptionismorecomplicated,theprinciplehasnotchanged,butthealgorithmisaligned.Insteadoftransformingletters.Knowingpastpasswordsisstillaverymeaningfulthing.
Encryptionmethod
Inasimpleverticallinechangecipher,theplaintextiswrittenhorizontallywithafixedwidthonapieceofchartpaper,andtheciphertextisreadintheverticaldirectionanddecryptedThatis,theciphertextiswrittenverticallyonthechartpaperwiththesamewidth,andthentheplaintextisreadouthorizontally.
Example
Forexample,thetranspositionwithaperiodofeistodividetheplaintextlettersintogroups,eachgroupofeletters,andthekeyisareplacementfof1,2,...e.ThenaccordingtotheformulaYi+ne=Xf(i)+ne,(wherei=1,...e;n=0,1,...),theplaintextX1X2X3...isencryptedintociphertextY1Y2Y3...Thedecryptionprocessiscarriedoutaccordingtothefollowingformula:Xj+ne=Yf-1(j)+ne,(wherej=1,...e;n=0,1...).Plaintext:COMPUTERCRAPHICSMAYBESLOWBUTATLEASTTIE'SEXPENSIVE
COMPUTERCR
APHICSMAYB
ESLOWBUTAT
LEASTITSEX
PENSIVE
Ciphertext:CAELPOPSEEMHLANPIOSSUCWTITSBIVEMUTERATSCYAERBTX
Other
Becausetheciphertextcharactersarethesameastheplaintextcharacters,thefrequencyanalysisoftheciphertextItwillrevealtheprobabilitiessimilartoEnglish.Thisgivesthecryptanalystagoodclue.Hecanusevarioustechniquestodeterminetheexactsequenceofletterstogettheplaintext.Theciphertextgreatlyenhancesthesecuritybychangingthepasswordtwice.
Althoughmodernpasswordsalsousetransposition,theyrequirealotofstorageandsometimesrequiremessagestobeofacertainlength,whichismoretroublesome.Insteadofapassword,itismuchmorecommonlyused.