Cryptography Forouzan

7/21/2019 Cryptography Forouzan

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30.1

Chapter 30Cryptography

Copyright The McGraw-Hill Companies, Inc. Permission required or reproduction or display.


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30.2

30-1 INTRODUCTION30-1 INTRODUCTION

Let us introduce the issues involved in cryptography.Let us introduce the issues involved in cryptography.

First, we need to define some terms; then we give someFirst, we need to define some terms; then we give some

taxonomies.taxonomies.

Definitions

Two Categories

Topics discussed in this section:To

pics discussed in this section:


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30.3

Figure 30.1 Cryptography components


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30.4

Figure 30.1 Cryptography components


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30.5

Figure 30.2 Categories of cryptography


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30.6

Figure 30.3 Symmetric!ey cryptography


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In symmetric-key cryptography, thesame key is used y the sender

!"or encryption#

and the recei$er !"or decryption#.

%he key is shared.

"ote


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Figure 30.4 #symmetric!ey cryptography


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Figure 30.5 $eys used in cryptography


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Figure 30.6 Comparison %etween two categories of cryptography


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30-2 SYMMETRIC-KEY CRYPTOGRAPHY30-2 SYMMETRIC-KEY CRYPTOGRAPHY

Symmetric!ey cryptography started thousands of yearsSymmetric!ey cryptography started thousands of yearsago when people needed to exchange secrets &forago when people needed to exchange secrets &for

example, in a war'. (e still mainly use symmetric!eyexample, in a war'. (e still mainly use symmetric!ey

cryptography in our networ! security. )owever, today*scryptography in our networ! security. )owever, today*s

ciphers are much more complex.ciphers are much more complex.

Traditional Ciphers

Simple odern Ciphersodern !ound Ciphers

ode of "peration

Topics discussed in this section:Topics discussed in this section:


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Figure 30.# Traditional ciphers


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( sustitution cipher rep)aces onesymo) *ith another.

"ote


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The following shows a plaintext and its correspondingciphertext. +s the cipher monoalpha%etic

-xample /.0

Solution

The cipher is pro%a%ly monoalpha%etic %ecause %oth

occurrences of L*s are encrypted as 1*s.


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%he shi"t cipher is sometimes re"erred to

as the Caesar cipher.


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4se the shift cipher with !ey 5 06 to encrypt the message

7)-LL1.8

Solution

We encrypt one character at a time. Each character is

shifted 15 characters down. Letter H is encrypted to W.

Letter E is encrypted to T. The first L is encrypted to A.The second L is also encrypted to A. And O is encrypted

to D. The cipher text is WTAAD.

-xample /.


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4se the shift cipher with !ey 5 06 to decrypt the message

7(T##9.8

Solution

We decrypt one character at a time. Each character is

shifted 15 characters up. Letter W is decrypted to H.

Letter T is decrypted to E. The first A is decrypted to L.The second A is decrypted to L. And, finally, D is

decrypted to O. The plaintext isHELLO.

-xample /.


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30.20

( transposition cipher reorders

!permutes# symo)s in a )ock o"

symo)s.


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30.21

Figure 30.$ Transposition cipher


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30.22

-ncrypt the message 7)-LL1 < 9-#=,8 using the !ey

shown in Figure /.>.

Solution

We first remove the spaces in the message. We then divide

the text into blocks of four characters. We add a bogus

character Z at the end of the third block. The result isHELL OMYD EARZ. We create a three-block ciphertext

ELHLMDOYAZER.

-xample /.6


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30.23

4sing -xample /.6, decrypt the message

7-L)L91


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30.24

Simple odern Ciphers


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30.25

Figure 30.% @1= cipher


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Figure 30.10 =otation cipher


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Figure 30.11 S%ox


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Figure 30.12 A%oxes: straight, expansion, and compression


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30.2'

odern =ound Ciphers


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Figure 30.13 9-S


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Figure 30.14 1ne round in 9-S ciphers


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30.32

Figure 30.15 9-S function


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30.33

Figure 30.16 Triple 9-S


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30.34

Ta&le 30.1 #-S configuration


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30.35

(+ has three di""erent con"igurations*ith respect to the numer o" rounds

and key sie.

"ote


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30.36

Figure 30.1# #-S


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30.37

Figure 30.1$ Structure of each round


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30.3&

Figure 30.1% odes of operation for %loc! ciphers


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30.3'

Figure 30.20 -lectronic Code Boo! mode &-CB'


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Figure 30.20 -lectronic Code Boo! mode &-CB'


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Figure 30.21 Cipher Bloc! Chaining mode &CBC'


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30.42

Figure 30.21 Cipher Bloc! Chaining mode &CBC'


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Figure 30.22 Cipher FeedBac! mode &CFB'


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30.44

Figure 30.22 Cipher FeedBac! mode &CFB'


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30.45

Figure 30.23 1utput FeedBac! mode &1FB'


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30.46

Figure 30.23 1utput FeedBac! mode &1FB'


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30.47

30-3 ASYMMETRIC-KEY CRYPTOGRAPHY30-3 ASYMMETRIC-KEY CRYPTOGRAPHY

#n asymmetric!ey &or pu%lic!ey' cipher uses two#n asymmetric!ey &or pu%lic!ey' cipher uses two!eys: one private and one pu%lic. (e discuss two!eys: one private and one pu%lic. (e discuss two

algorithms: =S# and 9iffie)ellman.algorithms: =S# and 9iffie)ellman.

!S'

Diffie()ellman

Topics discussed in this section:Topics discussed in this section:


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30.4&

Figure 30.24 =ivest Shamir #dleman &=S#'


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30.4'

In (, eand nare announced to thepu)ic/ dand are kept secret.


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30.50


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-xample /.


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Bo% chooses and 00 as p and D and calculates

n 5 x 00 5 . The value of 5 & E 0' &00 E 0' or ?/.

"ow he chooses two !eys, e and d. +f he chooses e to %e

0, then d is . "ow imagine #lice sends the plaintext 6

to Bo%. She uses the pu%lic !ey 0 to encrypt 6.

-xample /.

-xample /. &continued'


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30.53

-xample /. &continued'

Bo% receives the ciphertext 2? and uses the private !ey to decipher the ciphertext:

The plaintext 6sent %y #lice is received as plaintext 6%yBo%.


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30.54


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%he symmetric !shared# key in the

i""ie-e))man protoco) is

gymod p.

"ote

Arocedure


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Cryptography Forouzan