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MODERN I,OCKCIPHERS2 MODARN TREAMCIPHERS

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How many paddingbits must be added o a message f 100characters f 8-bit ASCII is used or encodingand the blockcipher accepts locksof 64 hits?

Sabsfilation or Transoasition ?

Suppose hat we havea block cipher wherer = 64. If thereare 10 1's in the ciphertext,how many trial-and-error testsdoes Eve need to do to recover the plaintext from theintercepted iphertext n eachofthe following cases?a. The crpher s designed sa substitutioncipher.b. Thecipher s designed s a transposition ipher.

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Blotk Ciphts ul Pumutariun Gtoups

Contponentsof a Moderw Block Cipher

C c rnpressionP-box

StraightP-box

ExpaDsionP-box

1 2 3

l./X,'., IT-T:rT-T-1 2 3 4 5

t 2 3 4 5

1 2 3 4 5t 2 3 4 5

t 2 3

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Components f a Modern Elock Cipher : S-BoxAn S-box (suhstitution box) can be thought of as aminiatare sabstitution ciph en

An S-box may or may not be invertible. In an inveftibleS-box, the number of input bits should be the same as thenumber of output bits.

Componentsof a Modem Block CipherExclusive-0rAn important component in most block ciphers is theexclu sive-o operatio n..-+--

Encryption| @. l- *-- ]---'m lo""rypt;ont Y t t : r t '---r--- ----r---Addition andsubtraction operations n the GF(2) Jield are.performedhy a single operation called the exclasive-or XOR).

4

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GF(?) and usess setaf 2nelenents. Theelementsn this setaren&it words.Addition

00 01 10 1 iMultiplication

000 lt 0

b1 2 D ! o0Aflel sLrifting

h.Aftcr shifting

000 lI Ol l

livadste Galois 1811 1832) tr{lentity: 0{} trdntit}. {tlThe Jive properties of the exclusive-or operation in theGF(29 fteld makes this operution a very interestingcomponent for use in a block cipher: closwte,essoci$tivity, eatnnt$t&tivily, axistence of identig\ andexistet ce of inverse.

ComltonentsaJ'a Mortern Block CipherCircular Shift

I .*,"*ati,ol

f--*f,'r*,i*i]

trdentityr}l

Before shilling Belbre shiliing

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Componentsof q Modem Block CipherSplit and Combine

ffir.F;F'p;l

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.FroductCiphersShannon introdaced the conceptofa prodact ciphen A product cipheris a complex cipher combiningsubstitution, permutation, andother components discussed inprevious sections.

Product CiphersDiffusionThe idea of diffusion is to hide the relationship betweenthe ciphertext and theplaintext.ConfusionThe idea of confusion is to hide the relationship betweenthe ciphertext and the key.RoundsDiffasion and confusion can be achieved using iteratedproduct ctphers where each teration is a combination ofS-boxes,P-boxes,and other components.

ClaudeShannon1916-2001)

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Horst Fehtel (I915 _ 1990)

&'eistel CipleersFeistel designed a very intelligent a.ndinteresting cipher that hus been usedordecades.A Feistel cipher csn huve three typesof components se f-inverti b e, nvertib e, andnowinvertihle.

I) ifferen i al C vptanu ys sEli Biham and Adi Shamir introduced heidea of dffirential cryptanalysis. his is achosen-p uintextattack.

Lllcryptio Ilncryprion

Eli Biham

Adi Shamir (1952 )

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Litzear CryptanalysisLinear ayptanalysis as presented byMitsuru Matsui in 1993. The analysisusesknown plaintext attacks.

k2klko

li,;", *'o. Il n= ! ' r an lt : - i : : I - tS'box

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Irisecure channel

What is the patte.rn n the ciphertext of a one-tine pad cipherin eaehof the fbllowing eases?a" The plaintert is made of n 0's,b. Theplaintert is madeof n 1's.c.Theplaintext s madeof alternating0's and 1's.d. The plafutert is * random stdng of blts,

& l r t i t

10

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Feedback shift register (FSR)

Output f)

Createa inear feedbaek hift registerwith 5 cells n which. tt : b t @b z @ n .

Createa linear feedhack hift registerwith 5 cells n whichht : &a@ t@ u.

Output()

Transitiorl 'bo ' + k i Il ' r -+ 6o iI b.' ...- b, Ilr:, :l::r:1:ilairi:,1iri:l:,:i:,:i'lt , , - .I t tn --+ p*)l

1 l

Feedback finction

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Createa inear feedback hift registerwith 4 cells n which&e $r S S* $how the value of output for 20 transitions(shifts)f theseeds(0001)2.Key streamgenerator

In a nonsynchronousstream cipher, each key in the keystreamdepends nprevioasplaintext or ciphertexL

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Encryptio1r