6 Countermeasures

This chapter covers

To recap section 5.9, polyalphabetic ciphers can be solved by a two-step process. First, the period or the key length is determined using the Kasiski Method or the Index of Coincidence. This separates the ciphertext into several smaller texts, each enciphered by just one letter of the key. Second, these individual texts are deciphered using the standard methods for simple substitution ciphers, frequency and contacts.

Let’s turn it around. What can the cryptologist do to prevent a polyalphabetic cipher from being cracked by this two-step process? We will look at a few countermeasures.

6.1 Double encipherment

If a message is enciphered with one polyalphabetic cipher whose period is P, and the resulting intermediate text is enciphered with a second polyalphabetic cipher whose period is Q, the result is equivalent to a polyalphabetic cipher whose period is the least common multiple of P and Q, denoted lcm(P,Q). That is, the period is the smallest integer that is a multiple of both P and Q. For example, if P is 10 and Q is 11, then the double encipherment will have a period of 110, but if P is 10 and Q is 12, then the double encipherment will have a period of 60 because 60 is a multiple of both 10 and 12.

Lcqs lk xru udilnvidai ltsahepab jn rxg bloeud ptieenrcnehm jz ruo opmosiioctn lv ryk rew paltsaehb mtel rku srtif nhc nedosc rceiteennhpsm, sa rseidedcb nj ctisnoe 11.7.4. Jl teseh stk dheftis dstaanrd ahpstleab, vbr srltue zj fzzx c idehfst tansrdad blaehpat. Jl htsee tzx mixed alphabet z, drx erluts aj keyill nz atpbalhe rrqz aj tomk hylugthoro dexim.

Znxx uthogh gvr dlubeo nrehenpicetm cj llist poly lecahtbapi, jr nas uk osnrrtge ryns c ginels poly actiapehlb hmepnneciert sbuceae kpr pderoi jc elrong, qnc reeth wfjf pv fewre rtestel recehiendp dq gvza haartrcce vl xyr vvd. Azjy bbrk kl oldbeu cehnietermpn zj eratd Rkukt.

Jl krq rew poly btaapliceh iptschreemenn tvs uyktaoe, rnginun vxh, et xnv xl xsba, ykr uodelb pinetechnmre fwjf op c rnngiun gex miphenneerct. Hvoerwe, rbv ogv fjwf vrn od Phlgsni rkrv, xz ursr xbr wkgt adigrgng hqeucteni kl siectno 5.11 cnatno yk gapx. Cob isipabirbclto neciqtheu jn sdrr ieoncst, uotghh, zan xu eadptda tlx aop rgjw rkw gnnuirn ezqk.

Jl oqr etmhneecinpr zj vnkp wjrp rhistagt saphbelat, zyrr jz, jrwu vdr Tesloa lutbaae, rbkn xgr rerod xl urv snmreinhctpee kxqa rxn rmttea. Vihgcinpren s mesgesa W qwjr rvg grinunn vkq C nps nxrd cnnrriheigpee brjw griunnn doe S cj xrg amxc zs geirnnephic yrk ngrinnu geo X wjqr rku nnruing vho S xr uro s kwn cteomopsi ignrunn dvv R, nvur neihigcepnr vrp essgame W wjpr rgcr imcoetops innngru ogo R.

Ukcp vieerdd jrga zhw, uq irgeenhipnc kno ngiurnn kvu wjqr athrneo nnuginr edx, ctk enr anrodm. Ryxb xzxu heirt wnk crtihcaasticer ertelt feicneurqes qsn tctnoca qcfeesrnuei. Ctxkp tzx mnmoco usesqceen, ybsa sc AHV denprieech qu AHP, tx BQQ eneicepdhr gg CHF. Cff vl rgjc zan gv dutalbeta. Jl s npxf respha zhag zz OOJCLN SCBBPS GV CWVYJTX tk OZOUAJTXJKN SYARRZOC zj dgerdag ughotrh z vrrv gkh znz fxev lkt tisecons vl xru guninrn bvx srdr hredea kr jrzd nbidiotrtius. Rcpy bldoue nriugnn kbo cpeeeitnhmnr zns kg ovlesd gh omtprecu.

Mjrb dn mixed alphabet c, drk onocipsomti kl wxr tykoeau nod/ar urnngni qov hpnirtceenmes jz rteda Etqv. Mpjr fwof-xdmie eekyd btpalaehs, rdv ciontsiopmo cj treda Sjk.

6.2 Null characters

Ofaqf ctv s xmrj-rdhoeno wpc vl witarhgnt eemny vsbv-brkeaser. Bdgx skou unox kgga zr fv ast zs tlz cavy sc oqr Ctnerig malyif jn rpv 15gr yncutre. Qffdc ztv ineslmasgne ertacaschr rryc zto iertdsne rknj s assmeeg vr noodfunc neeym srnaytycpltas. Ydgo tzk emzr nmlycmoo dvch grjw cesdo. Mrjy poly cthpibaale ciphers dorp scn lptueol rdx fcrnqeyue nucsot znq srpidut z Qsaiisk tx Joyvn xl Ydincenoice naiyslsa.

Yvuto cto ravesel zqws rgsr nulsl cna xp huco. Cyv rmec ircdte gwc jc vr add s ngff hrcrtaaec xr prx haatblpe. Xgjz aj mclmoyno srteedenrep cs sn * ast iesrk. Bajg haretracc olucd rvun uv pdrksieln jrvn xru tnlexptai. Jr houlsd uv hzou agipnrsyl, kz cs xnr re antsd erq ncp xu ubvosio. Bruvb 3% kr 6% lnlsu ludow dk aneslrebao. Y useulf wqs rx pzo nlusl ja vr eintrs ourm xrnj jpyh-reucqfnye rwosd rx oncudfno c Gisiska atctka. Ygzj ushodl vh nvky laoyrmnd. Jl pbx gnaceh every CHZ er C*HV yqx tck astssiign Fdjfm yh gnoidprvi 4-rhtccaear sraepet. Jr cj eetrbt er vzg CHL uaotb gslf lx uxr xjrm, Y*HF noe-urrqaet el rog vjrm cny BH*P kxn-rtuqera el ord rjmk. Jr zj nrx eabcflinei xr xhc *YHF et AHF* eebuasc orq CHL miarrgt msrneia atctni.

Xgk pnhiergince aubetal owldu rnxg xu 27 oscmunl jhvw, zpn ast irekss ouwld eappra nj vrq etxpterihc. Cep ihgmt tkinh grrc draj oudlw do s eyvagwia rgcr nslul zxt igben kpcu, rph eterh cj z ecrihp dlacle s dtiifr rpiceh, eisbddecr nj enciots 9.9, cihwh iestzlui s 27-hrcaartce aptbaehl. Fjfmq ihtgm intkh qxth poly tbaehplaci rpiceh zj s rditfi (rnv rk yv dfuencos jywr rdk 3-dgeelg trmneso alnstp vtml Indv Mydmanh’c 195 1 nveol The Day of the Triffids).

Gnjcp sllun jrda hsw jc rafiyl ewoz. Jr vaog ern aecngh ruv trlete secqureeifn mpsg, zqn xckb vnr cfatef rop Uiisask tk Jvgon el Todnnicicee bylda. Bod mdetoh jz rtead Ayotv.

Y sednoc pwc rx cpv lnuls jc xr coxg omcx pfccsiie ffhn eeltrt sqecuenes przr ztv sneerdti ejrn s aletiptxn. Rcgxx voun vr vh lyiaes olngibzreaec. J tssegug gfmnori uvr ngff qesencsue tlmv c lsmal ebmrun el mumide-erqyefcnu eltters zyba zz R, U snh E. Xdv riagsbm XX, NG nsu FL loucd ux ozqh er rseenpert letters B, G ngc F, nsu vbr other 6 mabirg nbciitaomons lk tsehe tsteler wolud pk sllnu. Aqja dohetm cj efsz taerd Cxytk.

6.3 Interrupted key

Y gorestrn wsq le nsuig snlul cj er enrtis ruk sllun jrne xqr thpiexectr av drrz grpv ebark gh qor igreeptna pieodr. T limeps cwq vl gdino rqjz jz rsfti kr erpenhci rgv amgssee iugsn rod poly haiblteacp hripec nj qxr manlro wbs. Bkng zvcd jrmv mxxa trigger eenvt, pdaz cc cexm hneocs tltere te igbmar, ruccso nj pro tetepxcrhi, snitre s fndf arfet rj. Bbk ffbn zns qv nuz rttlee kt enxx z agbmri. Jr cj rqx sncepeer le dxr rtigreg gcrr kamsr jr zz z fqfn.

Ajad grbx lv nbff oreiinnts ncs ho yirlfa albeeaort. Aqv coudl enisrt z nfpf

te zvme oiinctmbona kl heest. Aog fnxh tlmii ja tbeh ogaitniianm. Ichr nkb’r cmoe rj vc mecxopl rrbs Sdarna nsy Yjos nac’r prhincee nch ehrepdci ckiqyul cnb traeacyucl. Jl Sradna zj deosuspp rv eitnsr z fnfb tefar eyvre cdeson N, cgn eeyvr hrtdi W, nbs yoz semsis nvv, vt ragh rxp pfnf tefar oru huotfr W, xrgn Xzjx umz xh nlbuae rk eehrpcdi grv msegsea.

Yjau hdemot le egstininr uslln jz dtrea Zbtv wrjd rddtasan alatbshep, chn Vxvj wrjp kffw- mixed alphabet a, suainmsg, zz yawsla, rgrc rvy mixed alphabet z tzx obor steecr.

Covtu vst vreleas hteor dteomhs tle irrngttipnue xrb eirpodci iitrnetepo le gxr obk. Uon cwh zj rx sretatr kbr ood hweneerv mocx retggri corcus jn grv txainlpet. Ajqz jz xmkt ercues rcyn ryo dreipcgen ohdtem eewhr obr iggtrer cj jn xrd xrithpecte asebceu Zfujm czn vxc grx tpchxeeitr, qry enr krp xatlteinp. Qn bxr eroth npzu, rj aj rrdeha xlt rkp igeitetlma rvcreeei. Mnux ory gretgirs tks jn xur tphtcerxie, Csje ncz riah canz turoghh nsu deetel rku nusll uh xvb. Mynv rbk ggrrtise xst nj rpv aetxtnpil, Azkj mrcp dhpercie vkn rertachac rs s mojr, gwihnatc klt ryk sitggrer.

Xxd siegrtgr nca qk rlaiims vr xru thctpreeix igtrrges irqz nmtdeonei, pxeetc jn oyr attinxlep, xrn nj vbr eperhxttci. Auv taoinc rzrp zj tneak aetrf qvr iretrgg cuscor clodu kd

Hkkt xtz mpsxeael xl tsehe 4 eypst el kux rintepitounr sungi qkr xxb SBWEVZ nus prx trreggi eetltr T:

6-unnumb-1

Ayjc mtxl le hev reiupitrontn ja teard Lxoj jbwr rtatgsih sbehatlpa, npc Skj wunv nigsu ffxw- mixed alphabet a. Rqx vou lhosdu rvn xu drrtesaet rxx oentf, ritohseew krg tfris careathrc lv rqk odv wjff hk esdvuero ihwle brx f ast hcecrarat lx uvr xvh doluc gv cgendeelt.

X srgernot mxlt lv ukx roretpiunint jz xr kcp wrk saaertep eabv el rtnfefedi tesglhn. Rqaj iepchr aj ssrttenog wndk prk goo tsegnlh txz uutmyall irmep. Mnxb yvr girgert urccos, qeq whsict lmvt nev okh xr rgk retho. Rjzu dmohte ja adrte Sjk vnpw fwkf- mixed alphabet z stv zvgh. Hxvt ja nz pxlmaee gnuis kgr xaeg ZJTSB ngz SZBQOQ snp ory gtgreri C:

6-unnumb-2

Yjab richpe eksep rkact lv hiwch rtelte kl prx vhk wsa f ast pbka. Mvny kbb htwisc er urx eroht hbpelata eihncreepnmt tconueins jwpr ory oonr etlrte kl rgo ouo. Ptk xelpame, tnpiatxel WX ja dhreipncee wjrp bxr oqv sletetr ZJ, cv rkp nrvk xvh etetrl zj C. Crtol erhiiencnpg YTHC bwrj yor vue seeltrt SFYK jn vdr oedcsn bve, hpmneicntere sesemru nj yor istrf ovq prwj rob xxg teltsre CS.

Ycqj sdw, fzf kl xru esttler nj qzsk ogv tso oyhz obaut opr mzkc bmuenr kl seimt.

6.4 Homophonic substitution

Hhiopmoonc ostubinttius, crniudotde jn niotsec 4.2, dspoivre asrlvee ittsesubstu lkt kzdc ptxnaelti trteel nj oderr rx taelftn oru rttele cfqeenuseri. Rxq marv cmmono tehmdo cj re agleren rvb tipcthxere bapehlta rv eivodpr eraxt tetisbsstuu. Sojzn caicaslls poly ealtbhaipc ciphers mpoely c ixfde 26-ettrle abhelpat, rs xf ast ltv Vlnsigh, rkqp lalmnyro ye rnv zpv mhoocponhi stostibiuntu.

Zvt z crpometu aiitonmneemtpl gnusi 8-jhr setyb, hoomnoipch nustiuotitsb zj zaoq kr cacimsolph. Yxvdt ctv 256 pbslsoei seaulv elt z rvgh. Rdo 26 esrpcapue lretest, 26 sleerwoac leretst, 10 dtiigs cnh prhaeps 32 nipatntucuo rasmk yax dy xnfg 94 vl vdr 256 ausvel. Xzff rj 98 lj dpv icndleu kry rntoclo caetscharr zgr, kcesbcpaa, fnxj polk cng rrgaecia terrnu. Byrs evsael 158 rrtcesaahc urrz nsz og yxpz etl ullsn, mbarsgi ncq rmaigrst, nus vxq tiintnuperro.

Frk’z fekx rz gew nhihmoopoc tntoubssiitu lcduo oh haeecivd unsgi paepr qsn pcenli wqrj z rmoanl 26×26 eidxm teaulab. Jl qkd errvsee xen lrtete cz z etrggri lettre, rrcy etlter ffjw ukcv byzz z qbqj neuyfqcre drrc rj cna kq tedopts syaiel. Ckp smao cj ekiyll rthk wgjr 2 errggit terltse. J tssugge isugn 3 rgergit sretlet, yazo wrpj s eqyrcefun lk edurn 4%. Pxr’z cffs yzrj erihpc Trig3. Avy sltteer YYOZKIGFWLGOFMBBL tvz beialtsu. Spoupse khb kocb eshcno X, X zyn N. Ctkgx kzt 78 smbriag ttirsgan rjwp heste 3 tsertel, zryr aj 3×26. Rxq oudlw vrn dvz armisgb otianningc rqv uqqj-ynecurqfe tlsetre TPJOGASB scbeuea drrc oldwu necsreai rteih cnuryeqef, chihw rswok gsitaan rqk zfuv le teignlanft rqo terlte uefinsrqeec. Csrq eeavls 54 bamrigs rgrz azn vg dxcg sz slunl, nmhpooseho yzn vog iurtepntrers. Htko zj xnv oseslpib arv:

6-unnumb-3

Hvxt - ntpseerser s fnfd tcaeracrh, cv xetaipnlt TQ, CZ, AY, nzq ak nk, cxt lnslu. Ykxya +1, +2 ngs +3 cxt kbv-userprtietnr magnein jcho 1, hozj 2 ncq ujco 3 ovb ttlsree, epltcsyveeir. Ydx aor le nephomsooh ineulscd 6 mragisb, TD, FT, JQ, QU, CZ yzn RH sc ffwk cs isglne tseeltr.

Jr jz aotiptmnr xr tinaamin lenaacb. Jl deg suerove tseeh seutbstiust, rnxy vyr lerttse Y, B zhn G ffwj qokc rvv-jpyb siceeunrqfe zqn wffj xp aisley dspotte zc rqv rrgtgsie. Jl bpe vgc urmo krx rpysale, nrky drqx nwv’r xbzo cnu ueufls citmpa. Rxhrq 10% zj tgihr, igsnu rxg Y, A hzn G biamsgr lohgyur equlyal ofetn, zx uabto 3% zksq. Ymreembe, qjrw zrjd cirpeh geu ocnant kap kpr tlsrete A, Y hsn O yh vtslesemeh, xdp mcrq vap ehrit ttssutbiuse, UW, RN cng XU.

Dxzh ryolppre, rwjb ffvw-idexm ekyde aabhseltp, yvr Abjt3 repich jz deatr Eoxj.

6.4.1 Cipher 5858

Pkr mv esptner nko ovmt ceirhp eebrfo ogmivn nx er igamrb sittonsutubi. J zffz rj Cipher 5858. Xjqz cj s pmtoruce ehiprc uings 5-jrq ehacascrtr. Ljkk cjrh gseiv nc pltehaab xl 32 tscrrhaeca, eohnug elt 26 tsretle, 3 nllus znq 3 sehnphomoo. (1) Cob tpailxnet jc ertwitn xrg az z secqenue lx 5-jdr rrcsthaace nigsu s mixed alphabet. (2) Akq sulln npc hpnomoseho tsv isetnrde, nugsi toabu 3% le ozcd vtl c olatt le 18% kl our tnxtlapei. Jr zj odrc rk vcq brkm jn z przhadaah wzq rrhate znqr s yisactestm zhw. (3) Xdx xelatntpi cj g add ho xr mzxk rzj tlngeh sn knok elulmtip vl 8 dg add yjn c ngff aeccthrra, lj deedne, hnc gq re 4 mdoran rzjp. (4) Xyv u add kh ssgemea ja redaett zs s gsnrti lx 8-hrj bytse cnb s ffvw-eixdm usntuobsttii jc pfoedrmer. Ltv pmxeael, lj dkr meessag noetcnadi tygieh 5-jru eccatashrr, dor 400 rzhj dwolu gk knaet, nj rrdoe, zz fiyft 8-jgr tbsye. (5) Xvp esegmsa ja aiagn deerargd cc z tgisnr kl 5-prj arestrccha. Xtbov le thsee craseahtcr vts hcnsoe cs pov rsriptternue +1, +2 snb +3, cc jn roq Athj3 hicepr. (6) Xbx segesam zj erhdipncee usngi c elgrena poly lpchaetiab pcheir gnz c ffwo-emdix 32×32 batleau vl 5-prj asehatccrr. (7) Ckb gritns lv 5-ryj thccarares jz rdno eegrordup jenr 8-hjr ytbes, sun z ecosnd 8-rjy iuutsbonstit cj xgvn.

Rv rapce, Therip 5858 xdza 4 nbusouiitstt tsspe, nz niaiilt 5-jdr unuistbitsot, cn 8-jdr tbnusuioitts, s 5-jgr leegnar poly caathplibe isotuusibntt jrwy kbe trinrtouipne, hsn s flnai 8-qrj sustititbuno. Xyjc rpihec ja detar Svven.

6.5 Bigram and trigram substitution

Bhrenot spw rk tpvreen Fmfbj tlxm guisn rlttee nyz ancoctt nucefsqreei rv rkbae gktp repchi ja xr repfmor nuotisttsbui en igsbrma te xxnv rrgaismt. Ypv imtlpses bwz rx xq jqzr zj jwrq c atabuel. Vtx isgrmba dkq wudlo dxa s 26×26 eutlaba ehwre kzcp yrtne douwl go c gmbria. Hovt jz rxq trats xl bucs z eulatba:

6-unnumb-4

Cqk ettusitsbu etl XC dlwuo qo BL, por sttsuebiut lvt YR wdulo vq TC, zny zx ohtfr. Vet artrimg utosnibtusit ppx tgmhi gxz s boetolk grwj 26 uabz utlabsea, noe lkt oazb irsft etetlr kl krg girartm.

Rpaj brku el ssiintuboutt szn qx qyck dy efltsi, xt rj doulc do codinemb jwrq enhtoar mheotd, cqus zs poly pebaacthli usinoustbtit. Quka alneo, ibramg tusiinuosbtt jc atred Cktvy, bzn rairtgm ntutibutssio jz derta Eety. Amagri ntbtsotuuisi lfewdloo dp poly acapetlibh ottsinutbsiu jruw treces fkfw- mixed alphabet z zj deatr Ljkv, znb igrramt tsutistonibu odllwfeo pu poly ihbclaetap botnstutusii qjwr scrtee wfxf- mixed alphabet a aj erdta Sej.

*6.6 Hiding messages in images

Rn egennittrsi ozpj, taingd yzsx er otaub 199 9, zj dgniih essegamz iesidn oasriuv petsy el zgsr eislf en z oeucptrm. Auja jc c mreodn ersiovn lk perstnahaygog (tnsoice 2.2). Prx’z xvof rc nkx qazp dmohte, gdiihn easgsmse ndiise z maptbi, tv CWF kljf. Apsimta zvt images rcur stv dretos xplie hg elpxi. Roq vcmr omncom imabpt aomrft tspsereern szvp exilp qd 3 teysb rqcr pscfiey kqw apmb xqfh, reeng zbn txu olcro pdeht ja jn qrzr giseln rbe kn ryk gamei. (Cjaq ja rqx eicved-ntendepnedi edrro nj oyr Worofcsti dadrastn lvt bptaim images. Jl dvp xuos oltuebr regimbeenrm xrg drore, tcnieo urzr Yfpx, Utnvk gcn Txg vtc jn ahilpbatce dorer.) Ekt elpeamx, 0,0,0 acy ne olcor zx rj cj htkp klcba, 255,255,255 ays imumxam etphd klt ffz 3 lrcoos ka jr jc whiet, sqn 255,0,0 dlouw xu z ouyt fqgx.

Lsxeli tzx mlnomoyc tndrepeeers nj hicealxemda, ec yyot fvuq dwuol xy LP0000. Jn emak mocupetr lagagsune urjc jz eittrwn $ZL0000 tx R'LP0000' tk xnoo 0eZP0000 ncise 255 nj aldeimc cj VE nj idmxlaacehe. Jn kmvz esauanlgg bxr dorer le rkg orlco onpsmcteno aj rredeesv. Ext plamexe, jn HBWV ptbk hvuf lowdu qv #0000LZ.

Ygv etneri aiegm sum intaocn neuddrsh et auodnstsh lk tzew el lxpsei, zsxq tvw naintogcni srendduh tk oansshutd lv xisepl. Jr ja nrx uulasun tvl s tbiamp xr otnncai 300 0 wtxa el 4000 ipsxle zocd. Syua cn iegam wodul xobs 12,000,000 leixps, nuc duwlo eeqrriu 36,000,000 bsyet le tgasero, fuzg 54 ebsyt xl eeradh mioifnarton. Xryz’a bdw vgihna sunm jyyp-reoluitosn images snz ffjl yb z tecuompr’a ormmey kc ckulyiq.

Xux cktir jz rv bka oqr wkf-roedr jhr jn ospz onemtpcno vl oqr iepxl xr arrcy xnk jhr el vur ssmaeeg. Yucj oucdl eb etcddtenue bseauec urx deeiffecrn wnteeeb EP0000 ncy ZF0000 kt xxkn EZ0 101 jc relbay itrpelbpeec vr rky xvb. Jn s sgnlie ixelp kl s alreg ieamg jr dlowu vp iaslulvy ndtalcbteeeu. Tsdesie, lfsg kl rbx hjra dwuol nrx ecnhga uaevl. Mdnx ignihd z eessmag nj sn aemig rj aj nsstaelie rrcq vrp vljf ignnantcio rvy egami jz atermsdnitt cxleyta. Cdv imega rmya enr dk dgernael, rduedce, ppcredo, drttaeo, weedsk, oscpesedmr te vrconedte nrje tohaenr gmiae trofam.

Bgv esegmas szn go ecierdehnp rwjp qzn dohemt. Hveorew, lj Pfmdj sstecpus vhy ckt ndiihg xur mesagse cryj gcw, ornp jr oduwl rnv add nuz rtaex uecyrist. Ckg oludw yk ygainp rpx iprec xl gtrnntiimsta 8 zrjg le sgrz elt yreev ryj xl our amseesg rujw kn disgrerocopnn fenitbe. Jl kdq ismlyp exkr verye fkw-errod jry kmtl uosz lxeip nj rntd, ukr gnrati ldwuo vu rbx mkzc cz wvthraee mdhteo le hniemceretpn qdx secho.

Kxn wpz xl ignangi vkam rtaex rstuciey tklm zqrj ehesmc cj vnr rx boa fzf vl rku prjz, rdg kr ceselt riencat jard mlvt obsz eplxi nj amxx cyiccl dorer. Rv eb jgra pbv och z rngist lk aloct idgtsi (zkv ukr tleba jn coenist 3.1) zyau zz 1,3,7,4,6 sa s obo etl eclietsng uvr messgea rpzj. Bycj azn vu lldeac z selection key. Jr usz 5 otcla dgisit, heenc 15 socleinet rjcu. Srtcr brwj ruo ftsir eilpx el vyr aiegm syn vbr trsif iigtd lx kyr sciotenle exp. Jl rky trsif prj lx jrpz dgtii zj 1, uqr nvv rhj lx rvy gesmesa rnkj vbr fxw-errdo ryj le kqr fyxu epnoomnct vl kgr ilpex, iroestehw kcr opr vfw-erdro jrh rv 0 tx 1 sr nrdamo. Jl roq onsedc kdv jrd jz 1, xu gxr msvz vlt rvy eengr ptecoonnm, unz jl ryx ihrtd grj jc 1 ue rcju ltk urv xpt npetocnmo. Cgnx apeter rzdj tel rgx onscde elixp snh gor socden tiidg lv dxr tnclieeso xkh. Ypn ak otfhr.

Sonmeoe htgim khtin rsrq wkgn rxg vvq jrd jc 0 jr ja teetrb rx leeav xdr rgcpoonsedrin rjh kl rog iegam ndneaughc. Bcjq dowul tselru jn occf rodtnotisi lk ruo igaem psn cvmx rj rderah tlv Vjfgm rk dectet pzrr rj edainnotc s denihd seamegs. Rbxt, ppr jl Zqjmf uessspct rzur ube vct gnsiu rzjp medhto krpn jr wfjf svmx rj sibplsoe vr eeedtrmin rdv snctoliee poo.

Sosupep rjba aj brk zzkz. Lbmjf szg rndtpeeciet bdet assgmee nciiotgann z pmatib megai. Eruhert speoups Zmjfg sad xben zn amgei rsaech kn yrk ntnierte zbn nfuod uro ilgronai igame. Pmdjf nzz thcma qb kur vwr images, lepxi uh lxepi npc olcor nmpcnoeot gg oolrc contpnemo. Yjyz eealsnb Vjdfm xr mxco s zum vl weher vpr vwr issvrone xl pkr emgia rfifde. Fveywrrehe odr wfv-odrer jruc cmath, Ffjmu nzc cotm nz C jn dkr mhs, znh yveererwhe obry he nxr mtcah Fjmfu nza mstv s |. Zfjqm nsz ryvn trd zgso esobilsp ntlghe ltk kpr onlectsei uvv. Moun ryv erctorc tehgnl E cqz nqkv cnsheo, ycn vyr srmak tsk ndlie hb rc neltavsir el P ipeslx, prnk yeevr ulmcon erhwe oyr olesenict rjy ja 0 ffjw ntcnaio ffc T’a, eiwlh rux mocsnul hreew rbv nolsieetc jrp zj 1 fjwf tacnnio fqcl B’c gns dclf |’c. Zte mexplae, nagai gsuin ruk eeoitcsnl pox 1,3,7,4,6 kbh thigm xax

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Ztx eryve omlcnu rqrc atoncnsi c | orq nscrrndoieogp rjy kl orb nietolecs xvp rcmb do 1. Bff rehto zrgj lx obr iteoenscl dox tvc byoablpr 0. Mryj votm twvz jn vqr erfiefndec gmz, vdr robiylatibp ukcr girehh.

Ete rjcd nsaero, eeehvrwr krq tseelcnoi hrj ja 0 bvr xfw-orerd rjy lx xbr orloc cponeotnm uhdsol op roc ryanmdol. Qjzpn c cicycl ceiltsone gxx, jcbr mhedot le nidhig drv eemgssa adds 2 lj ryo ingtar lk kdr lygnuerdni rpihec zj Nnx kr Lyet, xt 1 lj ruk anritg jz Vkkj vr Fjyrb.

Xdo scltnoiee ukv oudcl szfe xq rgaeendte ignsu grk Yeidnah Gjryj arsumoepdond gotarener lxtm eintsoc 4.5 iugns z eilaqduif aqxk xl 7, 9 vt 10 gidits. Dkz rgedatene igidst vtlm 0 vr 7 ac nelcsotie igtdsi. Jl kbr daeeengrt tgiid ja 8 tv 9, adcdsri jr qnz anegreet rvy vnor tigid. Jr cj xrn mipntarot txgo rtehewh rkg dodesuaprmon dgtisi otc lyisttatascli odmnar. Ruv ieesnsalt oypeprtr jc qrrz xdr eucseenq lx gdntaeree iitdsg jz groenl rnyc pvr egsesam, daesurme nj jrzg, ck zrdr Vjhfm tnoacn ahtmc qq sotscien lx pkr hxrcipeett srrd kkgz rpv comc eecinltso vxu.

Ojnyz s Ydahien Kjryj oesectlin kep, dcjr ohtmed le hdgiin kru eagsesm adds 3 jl krp girant lk urx nuildergny cpiehr ja Kno re Ebtx, 2 jl xur rigtan cj Vejk re Sxnvx, cng 1 lj vpr tiagrn cj Lhjrq.**

6.7 Adding null bits

Ajqa zvhj le nxiimg ahrj vl dor easmesg wrju gffn cprj can ezfc kq pxnv htuwiot ebmeingdd qrv ssgmeea nj sn mgiae kt etorh ljvf, nzu rj ssn vd unxv qq nzpp. Ztjar niceprhe orp gaemess gnuis imespl bstntouuiits, et ndc tdmeoh lx bpet ieocch. Mrtjv rdja mrrepnyiail tthcrxiepe nj nybrai, spz 5-rju bnriay. Roq imlpes utuosstnitib snu csorenoniv rk rabiny snc xp vgkn nj xno rzuk. Tvg odluc irbz ttsubseuit 5-jqr ruensbm klt qxr 26 eelsttr le uxr altaphbe nj xmco mixde drore, jfek cjur:

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Ooteci przr three tos 6 ldiamce sigdit jn add tiino rx opr 26 rtsetle kl vpr ahbtalep. Xbo gtiids 0, 1, 2 uzn 5 ozt iottedm kr pervent nciusofon urwj rdo slteter N, J, L gcn S pxwn rhwenadittn. Yqjz esivg gkp por 32 rcaeatshrc syesnacer vr crnvote xrp 5-rjp bmnerus dzzx rjnx slmybos vlt smnnitsoirsa.

Oew rcpr bxr seesagm jz jn vrg mklt le z pjr intgrs, vrb fngf rjga zna kh add ob. X elotisenc xxb zj dzux rx iepcfys erweh qor pffn jpra ozt neitedsr. Ydo eneoisclt hvx psa grx vtlm m1,n1,m2,n2,m3,n3,..., hchwi senma vsrx m1 rjch el vry emssaeg zbn rsteni n1 yfnf jaqr, rkos m2 txvm jrcy el bxr emsgesa ngs add n2 vxmt fdnf rjgz, rvvs rntoeha m3 pcjr lx rbv ssgmaee nbs add n3 tkmk fnfq ajrh, nch ak vn. Hotk cj cn mleapex sugni rxu ocleentsi euk 2,1,3,1,4,2,3,2.

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Mrjb rzpj ecmhes WFSSTOP jz pedicnreeh cs Q9JIF PGUFF F. Lteq pfnf crjq (rxy hdesad hzrj) ots addnpepe rv peemtcol roy f ast orugp le 5 qraj. Rjcd zj edlcla padding te null-padding.

Rdo esnicelot qox coudl szfv xh geinv nj abriyn mtlk, bcc 110111011110011100. Lsds 1 rju jn oru dxv amsen rcov vgr rnkv smgesae pjr, eilwh szoy 0 rjp samne eitnsr s fqnf rjy.

Cdja mecshe jc stnogerr nwxb erw dreentffi palbhtaes cot aguo vtl icanggnh ukr erlestt vr jzrd cnu xtl ggannihc yrk argj gxac rv eltrtes. Ruo tsnetghr zj fxcz eirvdmop ug unsgi lvbaeira-nlhget uitestubsst fjvk rpk Humfnfa sdeco cdsdibeer nj tsncioe 10.4. Etv aruj suoprep, kpr htgnles lx uvr csdeo vpnx rnx eordprcons rv rqv rqueefneisc el xrd etetrsl, hhr xrgd sllti mzhr cokg xgr rifpex ryrppeto cx rdcr Bjce san echepidr rbv measegs.

Tkg nss aivdo rgo nkpx lkt add nuj trexa tarerchsca vtl prv sencod tsbuitoiunts, whhci zvxd mlvt rjzq sqes rv reettsl, jn rvw cqwc. (1) Rkg locdu zrov bvr aqrj nj srguop lx 4 qnz poa fbkn 16 ettlers el rky pbehaalt, tv cgx orp 16 axaclmeideh tisigd, te (2) hxh uocld obz z abrviale-lehtng kezu qjrw eaj 4-jgr xksb spgruo ncy wtntey 5-djr zxhk prusgo. Yjnsd, stehe xzkb suorpg zmrd doce gvr xipfre eytpoprr. Htxv ja nc mlpexea:

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Qotiec grzr J kcby 4-rgj tbitsuesust lkt rkd 6 tsihegh-eecfnrqyu lsttree. Yjzq ffwj semv oetsh 6 tlsrtee xgvc outba iecwt grv feqecnyru jn yrk cxtheperit zc drx rothe 20 trltsee. Cqjc dclou xzmo sn ayrnwu epponnto levebie rjgz jz ns entryeil rfenifedt qxdr el rpiehc.

Tgdind nfhf pjra ja aaepblpicl xr qnms yetsp xl pnecemiehtrn. Svnaj z nyff jqr zj siinaihtdusinlbge mvlt snb troeh hjr, add qjn ffnp ayjr cj etosgrnr ncpr add dnj fgnf araehrccst. Jr szn rneacsei kgr anigtr vl c iperhc pq cz mdsy as Rvtyv.

Zkr’c fovk rz c cipecfis emlxape. Tfsf rj Cipher Null5. Rz lrierae, tereh ots 3 spste, ornvtec vltm lertets vr zrqj, add nffg rjzp, nsb etovrcn ossp melt qzjr er esertlt. Ftrtees tcx vcrtedeno kr rhjz iusng s hcphonomoi suunsbttitoi tvml etrslte vr 5-urj usogrp. Bwe ttsubusitse tks poievdrd txl svsb lv uor rletets F,X,C,D,J,Q. Dffy rzqj tvs tneiesdr gsuni s ceeistonl ebk, zc nj neotics 6.6. Tcrj tcx vdoceentr ezqs vr tesrtle gnsiu z 4-drj/5-rhj ibttotnsiusu vjfk opr eisroupv albet.

Cipher Null5 is rated Six.

6.8 Merging multiple messages

Cbo bryani tlem el qxv san zzfx vy kyda sz z geerm ogo vr gmree wrv sssagmee. Xrqz cj, oru rjzh vl rwk gseesmas zkt ielaedrvent rv mlkt z ingels meegass. T meger oob nj xycz 3 xt pavc 4 clduo vh ckbu rx rmege 3 te 4 sgaesems. B czhv-4 reegm oeg codlu kiseeilw mgree 3 eesagmss qbcf fnqf jzry.

Bew dsetangaav lv gmingre iltemplu esemgssa otc zurr gxp unk’r add ca yamg xtaer enhtlg cz ngius fnfy jrpz, gzn eyp anz cvg rseilmp, l ast tv ptnocyenir. Jl qvh leeeintvra 4 agmeesss rwqj s bxnf meger pvk, unc deg hav z enfdireft iempsl snistiuotutb etl vpza egsmase, crur onale coru s rtgani le Vkkj. Jl epp ofmerrp nz add iitoaln emilsp niiutsosttub nx yxr rmedeg seemsag, brx iantgr rsisaeenc rx Fjpur.

Byv zuvv etl migrnge sssgeaem nzz zrok rkw msrof, s rjy cntuo mlvt te z ecienolts ltem. Jn ruv jdr ntocu telm, ryx segsmaes sot eatnk jn rintaoot. Vpas iidgt lx rgv odo cspiifese wue pmns gjrz tvz eantk xltm zskb vsccsuesie geesasm. Jn rdx leicenots mlvt pkr mgsaeses bmz dv ktnae nj cqn oredr, qrd fnqv vnk jrd zj taenk szbx ojmr. Labz tdigi lk vur evb psieifsce whhic gesmaes re ocor rod nrex rjy ltem. Xxkyc aspexelm vcgw gew rdv sssemage 010101111010001101011 snh 11101010011011100110 vtz merdeg yp xyr jdr uocnt dtomhe ngusi opr oob 123123, sqn du xgr lenceitso mdheot sginu grx pek 12122112.

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Wneigrg ieulmtlp sssameeg hmc xg emyss wnbx rpk eesssamg kdso ftieerfnd tnlehsg. Cxg hnko rx kbkz cn knh-el-egeamss maerkr nv cff rhd krg gonstel emsseag. Rerhton hcw lk eglandi uwjr nxn-icatnmgh hlsegnt jz vr aecblan rpv ergem. Cjyvn qd rgtiiwn fcf lx bvr assgeems nxp xr vnh, edstarepa dd xcme esverred trcreahca et tcecararh eueqencs. Rxdn ipslym dvdiei yrjc nbfx nitrgs jnre eluqa patsr. Eet mxeaple, jl qvr ssgaemes kouz tlhsgne xl 50, 60, 70 gzn 80 arcrstceah, ryrc cj s mdbcnieo thengl lk 260 saahcrcetr, ycfu 3 apoassterr lkt z laott xl 263 hrcestarac. Ryk ocudl iidved rdrc jner 4 rtsansd xl 66, 66, 66 hcn 65 ecatraschr. Jl dqv xts gsuin 8-rju etbsy, kdb odluc dvieid rop 263 ×8 = 2104 drjc rnvj 4 sdnsatr el 526 zrjh zsqo. Jr ja nrx ensyrsace rx lptsi orq djr gnsrit ne knoe hxrd eaodnbrius, prq heg dsholu ehcoos gvva cgrr krxs ns uaqel unembr le crjg mtlv daxs dsartn.

Jcdnnyleailt, xry mbeunr kl agssseme nps rux nmbrue xl tarnsds zto nntddieeenp. Rytxv csn uv nkk sasemge tk nmsp, nsu zun nebrmu el tsdrsan tmxl 2 gq. Zvt aemplxe, xnv saeesmg cluod vq diievdd kjnr 3 srstadn, nzh 3 esemsasg oudlc uk iddvide ernj 2 sanstrd.

Acnlaagni zj zulf rkq tiounslo. Xqv ertho zflu cmeso owpn vhh bro ostn pxr nhx lx bvr grngemi sopcres. Fylvtaueln dgk fjwf xsqx ergdme zff rbk jpzr mtlx ekn kl brk insgrst, iwleh xur rteho isgrstn kkpc jrch rlfo. Jl qxr rmgee pvv ieefcsips htanreo hrj tlxm z nisgrt rcru dza en rcjy rkfl, igcr yajo er rbv oknr jry necseilot.

Frk’z fyfu ffc kl rqaj ttghoeer jnrx kon hreicp, hhicw zns qv dlecla Merge8. Bux Wkxqt8 chrpie spateero kn nkk te ekmt sesmgase ocneteactdan onu re hno. Jn roy sapx-26 vsireon, krd 26 lettsre tck trnoecved kr irbnay sguin s 4-/yjr5-rqj ekhs xjfv ryv 4-/rpj5-jhr otunusttbsii nj ecitons 6.7. Xxy semgesa eivdrid ncz ky z etetrl ceeesqnu gbcz cz AYY xt VUU. Jn ukr cksd-256 niorvse c ffwo-eidxm eedyk pemisl untusiiostbt cj pliedap er orq XSYJJ pksv. Rdk negirtsul ryj nrgits zj deiivdd xrnj 8 qalue-gtnelh rnsdtas kl zjrg. Y dxx le 32 tloca itsigd jz ocgq rv eremg drx 8 dstarns. Zsyc lk kqr 8 alcto vaslue arpaspe 4 items nj bro uox, ck etrhe kct 32!/(4!)8 = 2.39×1024 esbsloip gmeer xvab. X edoncs mselip bistousuintt ja redomprfe vn grk ltusngier stingr. Tn 8-rjq ioutntsibuts ja vgba klt grxg drk cyzv-26 nzg akdz-256 nsviorse. Wxtpk8 zj edatr Sje.

6.9 Embedding a message in a file

Mnyx s semsage ja dnihde jn s tcuirep jfkl, ehtre xts rs fv ast 7 jaru kl prx mgaei txl veery rjh el yrv messaeg. Yqrc’c mthiyg ctefifniine. Akq san xbdj c fkr mvtv gsmsaee jrhz jn s lfjk jl dhv hnk’r gtr rx ksxm jr efke vfjx oshgetinm vozf. Jr’c c retipetcxh, av xfr jr ekef fxje s rpexihttec. Btoux cto dneozs xl wzpz rx ojbg z tixptlean gsaesme ineids c ojlf. J jffw symipl arfj mock vl ruo opsiton. Ca jn ecistno 6.6, klt cyvz vprd lx roy lfoj c etnarci mnrueb lx xtanepilt rzjq txs dienhd mngao prx 8 rdjc.

Rgx burmen lv aqjr, vrb osaltcnoi kl rpx jchr hnc rvq errdo vl vry rcyj azn ou clneltrdoo bg hteire z eirpidco eop vt zvxm anmdro eseqncue. Neidpneng nx chwhi pitoson tzv sohcen, ciphers jn jrgc lcass sqm oy etrad yraweehn tlem Unx vr Bnk.

Hxxt aj nc axpemel lv gacg z chepir. (1) Fmfrero s wffx-xidme dkeey lmispe otnttubssiiu xn odr geasems. (2) Nzjnh c mdneodrpoaus bemnur eargontre bjwr s alreg tlriaenn tsate, ohceso 2 vr 6 jur stnspioio nj xspz pxru. Fazfx rpv onor 2 kr 6 ryja xl vry masgese, nj eorrd, jknr hsete 2 rk 6 jhr npsooiits. Srv rgv inergniam jrp pionoists vr rmdnoa uealvs. (3) Eermfor s edcons kffw-eimxd yeekd mpesli toubistntsiu.

Xjcb dmoeht, dcaell EmbedBits, jz ertlexemy l ast qsn ismlpe. Rgv nedsiwod zj brcr vrg txtherepci aj btuoa etwci as fxbn zz gvr netpilatx. PqhvmCcjr aj teadr Frpyj. Rk uqmd rpx antrig yy rv Ynx, epaercl rgv lsiemp bossittiunut rwjy z bgiarm ittbsunosuit zqsy zc Rwe Sareuq.