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I2

USEOFMODERN LOCKCIPHERSUSEOFMODBRN TREAMCIPHPRS

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Sywwari*kq eneip.&evmeat*n be dp&e$rfug mqd.em lack eiphew"Mode.sqf, rye.wtlalt ftqve bee&devised e eneip.heteet af.'any skeemployingeither DESorAES

E: Eucryption D: DecryptioDPi: Plaintextblock i Ci: Ciphellext block iK: Secret ey

Fb'..-] F;;-lr{ql "' r-,{41Tffil

Fbt"-l Fbt*l Fu',''lI _ l - IK-,1lK-ml ...*-'lglM M &DecryptioqDcryptioa

Etor PrcpagationA single bit eftor in trunsrr.issioncqnblock. Eoweve4 the error doesnot havecteate errots I severul in the correspotdWan! effect on the other blocks,

Elicfionic codebook ECB) mode

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Ciphertcxl StculingA techniqae called cipheftert stualing (CTS) can make it possible to useECB modewithoutpadding. n this technique he ast twoplainturt blocks," *_, or " *, are encrypted differently and out of order, asshown below,flssuming thst PN_, has n bits and P* has m bits, where m ! n .

: : -X=Er(P,1.- ) -+ CA/ = heador(XJY = P,vlrlll.,-,, {X) -+ Cru- = E6 (Y)

Cipher block chaining (CBC) modeE:Eicryptioi D:Decryltiol!: ?lai tex.l)lock Cr: Ciphertext lockK: Secrctkey IV: Initialvecior Co)

Encryplion DeclwtionError Prcpag.ttiortIt' CBC mode, a single bit etor in cipheriext block C - duting bqflsmission may createleftor in most bits in plaintext block P .duting .lecryption.

DecIWtion

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Ciphertext StealingThe ciphertexl stealing tuchnique described for ECB mode can also beapplied to CBC mode, as shown belaw,

The head anction is the same as described in ECB mode; the pad functioninserts 0's.

Eitcryption in cipher feedback (CFB) modeE: Encryption D: Decrlption S;:Shift isterPi: Plaintextblock i Cr: Ciphertext lock T,:Te$porary egisterK: Sesetkey IV: Initial vector S1)

li

Pz CzEncryption

In CFB mode,enciphermentand decipherment se he encryption unctionofthe underlying block cipher.

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KEI- geoemtor

Insecure haorel

Enctyption in output eedback (OFB) mode

Decry?don

0 : tncryt)lio! 1) : Decryyion S,:Shift&gisterP,:Plrinrcxtblod( q: Cifhertexxlocki 1,: Teopoturyegi$.rK: S!'crcl l:y IV; tdii{l wct4r(Sl)l rv . r - "__"_-__-- '_ '_ "

Ed-'WDffrynho'r

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OFB as a Stean CiohetKey generatoi

(r bits) ;

PiDecryptioD

In the couater (CTR) mode, there is noleedbach Thepseudorandomnessn the ke!strcsm is achievedusing a counter.

E : Encr,?tion IV: Initializatioo vcto1P,: Plairrtext lock Ci: Cjphenext lockK: Secret ey kr : Encryptionkey t

Eacrwtion

Key generator

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Coanter (CTR) mode qs d stteam cipher

Encry?tion

Conparisott of Different )Wotles

Key genefator

Decryption

Key Senerator

Su, u?fln' ol ope .1 iotl Ik4l c s

Mada ' aihch ,r-birblock s elcryp{ed ndependeorlyitlrtbes$rc cipherke),. BlockciltherSAnres 6CB. bureiichbloak s fifst exckrsire-nredwith thepreviolscipl1crtcxt. IllockcrpherFach,*-bil lock s exch$ive-oredith itr r-bit key-vhich ispn|tof prcviousiipher exl Skearnciphel f < t lSatrc rsCliB. hul hosllitl rcgisteri$$pdnlcd y $cprevious ,bit kcy. Srelllnc\)he. r S r lSaoeasOFB,b{t 0 corrteris rsodi stead fl rhift.eliislcf. S||clllrrciplrcl

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The dea of RC4 stream cipherStutedd ky initialiraionw - . wsl0lsllt st?l sl2J5lw - . &K[oiK{rlxl2l Klls5l

lnitlal stah!iEim(s1io

Key

InitiulizationInitialization is done n two steps:

Key Streott GenerarionThe keys n the key strcam aregenerated,one b! ohe,

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To show the randomness of the stream key, we use a secret keywith all bytes set to 0.The key stream or 20valuesof * is (222,24, 137, 65,163, 55,93, 58, 138,6, 30, 103,87, ll0, 146, tog, 199, 26, 127, 163).

A5/1 (a member of the A5 family of ciph*s) is used in the Global Systemfor Mobile Commanication (GSM), a network for mobile telephonecommunication,.

Plaintext frame Cipheftexl fra.treEncryption

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Key Generator45,4 ases hree LFSRs with 19, 22, and 23 bits.

At a point of time the clockingbits ar 1, 0, and 1. WhichLFSR is clocked shifted)?

En crypti on Dec yptio nThe bit streams created rom the key generator are buffered to form a 228-bit key that is exclasive-ored with the plaintext frame to create theciphertext frame. Encryption/decryption is done one rame dt a time-

Note: The three ed boxesare used n the maioritv linrction

LFSR: l9 bns xle .t ' +l +; + t)

LFSR2: 22 bits 12+n + 1)

LFSR3i 23bits 13 +.rr5 + .r + 1)

1 0