Chapter 1

Introduction

In today’s information age, communication plays a very impor-

tant role and has contributed heavily to the growth of technology.

The Electronic security has increasingly involved in making com-

munication more prevalent and robust. Therefore, a mechanism

is a need for to assure the security and privacy of information

that is sent over the electronic communication media. Whether

the communication media is wired or wireless, it needs to be pro-

tected from the unauthorized access of information. The method

of transforming the original information into an unreadable format

is called Encryption and the reverse process is called Decryption

of information. The study of encryption and decryption is known

as Cryptography.

Cryptography involves the study and the applications of the

principles and techniques by which the information is rendered un-

intelligible to all but the intend to receive. On the other hand, the

1

Cryptanalysis is the science and art of solving cryptosystems to re-

cover the unintelligible information. The computer security mainly

consists of three parts namely; data confidentiality, data integrity

and data authenticity. The data confidentiality is the protection

of data from unauthorized disclosure. The data integrity is the as-

surance that the data received are exactly as sent by an authorized

entity. The authentication is the assurance that the communica-

tion entity is the one that it claims to be. The present day Cryp-

tography involves three distinct mechanisms namely; symmetric

key encipherment, asymmetric key encipherment and hashing.

1.1 History of Cryptography

Egyptain Hieroglyphs (1900 BC): This was one of the first

known incidences of cryptography (Fig.1.1). A scribe used non-

standard hieroglyphs in an inscription. From the Greek meaning

’sacred writing’ was the picture language that was used often to

decorate temples and monuments. It could be written with pen

and ink on papyrus, or painted or carved onto stone. It was care-

fully drawn to make the signs as accurate as possible. Hieroglyphs

were used to write the ancient Egyptian language. In the beginning

hieroglyphic signs were used to keep records of the king’s posses-

sions. Scribes could easily make these records by drawing a picture

of a cow or a boat followed by a number. But as the language be-

2

came more complex more pictures were needed. Eventually the

language consisted of more than 750 individual signs.

Figure 1.1: Egyptian Hieroglyphs

Mesopotamian Tablet (1500 BC): A 3”×2” Mesopotamian

tablet contained an enciphered formula for making pottery glaze

(Fig.1.2). Cuneiform signs were used in the least common syllabic

values to attempt to hide secrets of the formula. Pictograms, or

drawings representing actual things, were the basis for cuneiform

writing. Early pictograms resembled the objects they represented,

but through repeated use over the time they began to look simpler

and even abstract. These marks eventually became wedge-shaped

and could convey sounds or abstract concepts.

Atbash Cipher(600 BC-500 BC): Hebrew scribes writing

down the book of Jeremiah used a reverse-alphabet, simple substi-

3

Figure 1.2: Mesopotamian Tablet

tution cipher known as the Atbash cipher (Fig.1.3). Many names

of people and places are believed to have been deliberately ob-

scured in the Hebrew Bible using this cipher. The Atbash cipher

is a Hebrew code which substitutes the first letter of the alphabet

for the last and the second letter for the second last, and so on.

This cipher is one of the few used in the Hebrew language. The

cipher itself, Atbash, is very similar to the substitution cipher. A

substitution cipher is one where each letter of the alphabet actu-

ally represents another letter. In the case of the Atbash cipher, the

first letter of the alphabet is substituted for the last, the second

for the second last and so on. i.e., for us in English the letter ’A’

4

becomes ’Z’, the letter ’B’ becomes ’Y’, the letter ’C’ becomes ’X’,

and so on. Atbash gets it’s name from the fact that in the cipher,

A becomes tav (the last), B becomes shin (one before last), and

so on.

Plain text: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Cipher text: ZYXWVUTSRQPONMLKJIHGFEDCBA

Figure 1.3: Atbash Cipher

Greek Skytale (486 BC): Ancient Greeks invented the ’Sky-

tale’ (rhymes with Italy), which was a stick wrapped with narrow

strips of papyrus, leather, or parchment. The message was written

on the wrapping; then the strip was removed and passed to the

messenger (Fig.1.4). Only if the receiver had the same size tube

5

would they be able to read the message. From indirect evidence,

the skytale was first mentioned by the Greek poet Archilochus who

lived in the 7th century BC. Other Greek and Roman writers dur-

ing the following centuries also mentioned it, but it was not until

Apollonius of Rhodes (middle of the 3rd century BC) that a clear

indication of its use as a cryptographic device appeared. A de-

scription of how it operated was not known from before Plutarch.

Mestrius Plutarch was a Greek historian/ biographer and essayist.

Figure 1.4: Greek Skytale

Frequency Analysis (1000AD): Frequency Analysis led

6

to techniques for breaking mono alphabetic substitution ciphers.

Most likely motivated due to textual analysis of the Koran. It

has been suggested that the close textual study of the Qur’an first

brought to light that Arabic has a characteristic letter frequency.

Its use spread, and was so widely used by European states by the

Renaissance that several schemes were invented by cryptographers

to defeat it. These included homophones, polyalphabetic substitu-

tion and polygraphic substitution schemes. The frequency analysis

was based on the fact that in any given stretch of a language, let-

ters and combinations of letters occur with varying frequencies

(Fig.1.5). In the English language for example, ’E’ is the most

common letter, while ’X’ is rare.

Leon Alberti (1466): Leon Alberti invented the cipher disk

and cryptographic key. Alberti’s cipher disk was polyalphabetic,

meaning that a new alphabet could be created each time by turn-

ing the disk. This type of disk was the only method of using this

type of cipher until the 16th century. Alberti thought his cipher

was unbreakable. This assumption was based on his inquiries into

frequency analysis, which was the most effective method of deci-

phering mono alphabetic cryptograms. Given enough crypto text,

one could use the frequency of the letters in reference to a normal

distribution to find the shift and solve the cryptogram. This sys-

tem failed to solve polyalphabetic cryptograms, however, since the

7

Figure 1.5: Frequency Analysis

letter distribution is garbled.

Vigenere Cipher (1587): The Vigenere Cipher is polyalpha-

betic, meaning that instead of there being a one-to-one relationship

between each letter and its substitute, there is a one-to-many rela-

tionship between each letter and its substitutes. The user chooses

a keyword and repeats it until it matches the length of the plain

text(Table1.1).

Louis XIV The Great Cipher (1626): A nomenclature

cipher developed by Antoine and Bonaventure Rossignol (Fig.1.6).

Each number stood for a French syllable rather than single letters.

The Great Cipher was used to encrypt the King’s most secret mes-

8

sages. In fact, The Man in the Iron Mask’s identity was protected

by the Great Cipher. The Great Cipher was not broken for two

centuries when Commandant Etienne Bazeries was able to take the

most frequent occurring numbers to decipher one common word,

less enemies.

The Morse Code (1763): The Telegraph showed that, elec-

tro statically generated signals which stood for letters of the al-

phabet could be sent a long way through a wire with the circuit

being completed through the Earth. The original telegraph used

26 wires; one for each letter of the alphabet. Samuel Morse creates

Morse code: Morse code represents letters, numbers and punctu-

ation marks by means of a code signal sent intermittently. This

Table 1.1: Vigenere Cipher

A B C D E F G H . . . Y Z

A A B C D E F G H . . . Y Z

B B C D E F G H I . . . Z A

C C D E F G H I J . . . A B

D D E F G H I J K . . . B C

E E F G H I J K L . . . C D

......

......

......

......

......

......

Y Y Z A B C D E F . . . W X

Z Z B C D E F G H . . . X Y

9

Figure 1.6: LouisXIV The Great Cipher

is an early form of digital communication. It uses two states ’on’

and ’off’, composed into five symbols: dit(’), dah(-), short gap be-

tween letters, medium gap between words and long gap between

sentences. Morse code differed from the telegraph in that it sent

code for each letter on a single wire rather than a wire for each

letter. In 1863, the European form of Morse code was created.

Kasiski breaks Vigenere Cipher (1863): Prussian major

named Kasiski proposed a method for breaking a Vigenere cipher

that consisted of finding the length of the keyword and then divid-

ing the message into that many simple substitution cryptograms.

Frequency analysis could then be used to solve the resulting simple

10

substitutions.

Zimmerman Telegram (1917): The Zimmerman telegram

was a secret telegram which included proposals for a German al-

liance with Mexico. The telegram was intercepted and decrypted

by the British Government (Fig.1.7).

Figure 1.7: Zimmerman Telegram

The German ’ADFGVX’ (1918): The German ADFGVX

cipher was the first cipher used by the German Army during World

War I. This was a fractioning transposition cipher which combined

a modified Polybius square with a single columnar transposition

used to encode a 36 letter alphabet (26 letters plus 10 digits) (Table

1.2).

11

The Enigma (1918): Arthur Scherbius designed the Enigma

- a device which allowed businesses to communicate confidential

documents without having to resort to clumsy and slow codebooks.

The device consisted of many rotors turning on a common axis.

The rotors had numbers 1 through 26 marked on the edge, or the

alphabet A-Z, and were equipped with 26 electrical contacts (one

for each letter of the alphabet) so that when a letter was pressed,

the output would depend on the position of the rotor and its cross

wiring. Within the same year, the Enigma was put to use; most

famously by Nazi Germany before and during World War II.

The World War II (1937 - 1945): The Navajo code talk-

ers have been credited with saving countless lives and hastening

the end of the war. The code talkers primary job was to talk

and transmit information on tactics, troop movements, orders and

other vital battlefield information via telegraphs and radios in their

Table 1.2: The Germen ’ADFGVX’

A D F G V X

A S U B J E C

D T A D F G H

F I K L M N O

G P Q R V W X

V Y Z 0 1 2 3

X 4 5 6 7 8 9

12

native dialect. A major advantage of the code talker system was

its speed. The method of using Morse code often took hours where

as, the Navajos handled a message in minutes. It has been said

that if was not for the Navajo code talker’s, the Marines would

have never taken Iwo Jima. The Navajo’s unwritten language was

understood by fewer than 30 non-Navajo’s at the time of World

War II. The size and complexity of the language made the code

extremely difficult to comprehend, much less decipher. It was not

until 1968 that the code became declassified by the US Govern-

ment.

Lucifer (1971): Horst Feistel created Lucifer at IBM’s, Thomas

J. Watson Laboratory. Lucifer was the name given to several of

the earliest civilian block ciphers and was a direct precursor to the

Data Encryption Standard.

Cryptographic HASH of passwords (1975): Hash algo-

rithms are typically used to provide a digital fingerprint of a file’s

contents to ensure that the file has not been altered by an intruder

or virus. They generally help to preserve the integrity of a file.

1.2 Computer Security

With the introduction of the computer, the need for automated

tools for protecting files and other information stored on the com-

puter became evident. This is especially the case for a shared

13

system, such as a time-sharing system, and the need is even more

acute for systems that can be accessed over a public telephone

network, data network, or the Internet. The collection of tools de-

signed to protect data and to thwart hackers is computer security.

In symmetric encipherment or secret key cryptography, an en-

tity A can send a message to another entity B, over an insecure

channel with the assumption that an adversary X cannot under-

stand the contents of the message by simply eavesdropping over

the channel. A encrypts the message using encryption algorithm;

B decrypts the message using a decryption algorithm. Both of

them use a single secret key. This system is used for one-to-one

communication.

A modern symmetric key block cipher encrypts an n-bit block

of plain text or decrypts an n-bit block of cipher text. Encryption

or decryption algorithm uses a k-bit secret key. The decryption

algorithm must be inverse of the encryption algorithm and both

operations must use the same secret key.

A symmetric encryption scheme has five ingredients:

• Plain text: This is the original intelligible message or data

that is fed into the algorithm as input.

• Encryption algorithm: The encryption algorithm performs

various substitutions and transformations on the plain text.

14

• Secret key: The secret key is also an input to the encryption

algorithm. The algorithm will produce a different output de-

pending on the specific key being used at the time. The exact

substitution and transformations performed by the algorithm

depends on the key.

• Cipher text: This is the scrambled message produced as out-

put. It depends on the plain text and the secret key. For a

given message, two different keys will produce two different

cipher texts. The cipher text is an apparently random stream

of data and is unintelligible.

• Decryption algorithm: This is essentially the encryption al-

gorithm that does inverse operation. It produces the original

message by using the cipher text and the secret key.

1.2.1 Classical Ciphers

They are classified into two types, namely; Substitution Cipher

and Transposition Cipher.

Substitution Ciphers

• Additive cipher or Shift cipher or Ceaser cipher: It involves in

replacing each letter of the alphabet with the letter standing

some places further down the alphabet.

Encryption algorithm is C ≡ P + B mod N and decryption

15

algorithm is P ≡ C − B mod N where P - Plaintext, C -

Ciphertext, B - secret key and N - number of alphabets in

cipher.

• Multiplication cipher or Linear transformation: It uses the

substitution as:

Encryption algorithm: C ≡ A ×P mod N and Decryption

algorithm: P ≡ A−1 ×C mod N where A - encryption key

and A−1 - decryption key.

• Affine Transformation: It uses the transformation as :

Encryption algorithm: C ≡ A×P+B mod N and decryption

algorithm: P ≡ A′ × C + B′ mod N , where A and B are

encryption Keys A’ = A−1 and B’ = −A−1B are decryption

keys.

• Mono Alphabetic Substitution Cipher: In this method a map-

ping is created between each plain text character and the cor-

responding cipher text character.

• Polyalphabetic cipher: a) Vigener cipher: b) Beaufort Cipher:

In Polyalphabetic substitution each occurrences of a character

may have different substitute. The relationship between the

characters in the plain text and a character in the cipher text

is one - to - many. Polyalphabetic ciphers have the advantage

of hiding the letter frequency of the underlying language.

16

• Play fair cipher: Multiple letter encryption cipher, which

treats digram in the plain text as single units and translate

these units into cipher text digram.

• Hill cipher or Enciphering Matrices: In Hill cipher, the key is

a square matrix of size m ×m in which m is the size of the

block. Let one block of the plain text be P ≡ P1, P2, · · · , Pm

then the corresponding cipher text be C = C1, C2, · · · , Cm

The encryption algorithm is C ≡ K ×P mod N , where K

is a m × m enciphering key matrix, the necessary condition

for the key matrix in the Hill cipher is that it must have a

multiplicative inverse.

• Auto key cipher: In this cipher the key is stream of strokes,

in which each sub key is used to encrypt the corresponding

character in the plain text.

Encryption algorithm: Ci ≡ Pi + Pi+1 mod N , Co ≡ Po +

K mod N . Decryption algorithm: Po ≡ Co−K mod N , Pi ≡

Ci − Pi−1 mod N .

• One time pad: Shannon has shown that perfect secrecy can

be achieved if each plain text symbol is encrypted with a key

randomly chosen from a key domain. This idea is used in a ci-

pher called one time pad, invented by Vernam. In this cipher,

the key has the same length as the plain text and is chosen

17

completely random. The main difficulty in implementing this

cipher is that the exchanging of key becomes difficult.

Transposition cipher

Transposition cipher reorders the letters or symbols in a predeter-

mined order.

• Keyless Transposition cipher: There are two methods in Key-

less Transposition cipher. In the first method, the text is

written into a table column by column and then transmitted

row by row. In the second method, the text is written into the

table row by row and then transmitted column by column.

• Keyed Transposition cipher: Divide the plain text into groups

of predetermined size, called blocks and then use a key to

permute the characters in each block separately.

• Combining two approaches: First the text is written into a ta-

ble row by row. Second the permutation is done by recording

the columns, third the new table is read column by column.

They are also called column transmission cipher.

• Double Transposition cipher: This can make the job of crypt-

analysts more difficult. The algorithm is repeated twice with

a different key or the same key.

18

1.2.2 The modern Symmetric Cipher

Fiestel Cipher:

The inputs to the encryption algorithm of Fiestel cipher are a

plain text of length 2w bits and a key Kbits . The plain text is

divided into two halves L0 and R0. The two halves of the data pass

through n rounds of processing and then combine to produce the

cipher text block. Each round i has inputs Li−1 and Ri−1, derived

from the previous round, as well as a sub key Ki, derived from the

key K. In general, the sub keys are different from K and from

each other. The parameters of Fiestel cipher are: Block size =

32/64/128 bits, Key size = 32/64/128 bits, Number of rounds =

16.

Data Encryption Standard (DES):

The most widely used encryption scheme is based on the DES

adopted in 1977 by National Bureau of Standards (now the Na-

tional Institute of Standards and Technology (NIST)), as Federal

Information Processing Standard 46 (FIPS PUB 46) The param-

eters of DES cipher are: Block size = 64-bits, Key size = 56-bits,

Number of rounds = 16. Since the key size is 56-bits it is possi-

ble to break DES. To make the key size large, Triple DES is used

where the key size is 168- bits.

19

Advanced Encryption Standard (AES):

In 1997 NIST called for papers for a replacement of DES. The NIST

specifications required a block size of 128 bits and three different

key sizes 128, 192 and 256 bits and AES must be an open algo-

rithm, available to the public worldwide. The announcement was

made internationally to solicit resources from all over the world.

The criteria defined by the NIST for selecting AES fall into three

areas:

• Security: The main emphasis was security. This criteria is

focused on the resistance to cryptanalysis attacks other than

Brute-force attacks.

• Cost: Covers the computational efficiency and storage re-

quirement for different implementations such as hardware,

software or smart cards.

• Implementation: The algorithm must have flexibility and sim-

plicity that is implementation must be possible on any plat-

forms.

After the first AES candidate conference, NIST announced that

15 out of 21 received algorithms had met the requirements and

have been selected as the first candidates in Aug 1998. After the

second AES candidate conference which was held in Rome, NIST

announced 5 out of 21 candidates - MARS, RC-6, Rijndael, Serpent

20

and Two fish were selected as finalists in August 1999. After the

third AES candidate conference, NIST announced that Rijndael,

designed by Belgium researchers Dr Joan Daemon and Dr Vincent

Rijmen, was elected as Advanced Encryption Standard in October

2000. In Feb 2001, NIST announced that the draft of the Federal

Information Processing standard (FIPS) was available for public

review and comments. Finally, AES was published as FIPS 197

in the Federal Register in December 2001. As though the AES

algorithm is resistant against algebraic attacks like differential and

linear cryptanalysis, it may have threat from XSL attack. This is

because the S-box used in AES is a static one. By making the

S-box as key dependent dynamic S-box, the XSL attack becomes

very difficult. If the key size increases Brute-force attack also needs

more time. The AES algorithm is discussed in detail in Chapter 4.

Asymmetric or public key cryptography: In this system there

are two keys namely; public key and private key. To send a secure

message to B, A first enciphers the message using B’s public key.

To decrypt the message B uses his own private key. This system

is used for one-to-many or many-to-one communication.

Hashing: In hashing, a fixed length message digest is created

out of the variable length message. The digest is normally much

smaller than the message (128 bits, 256 bits or 512 bits normally),

both the message and the digest are sent to B. Hashing is used for

21

providing the data integrity.

Shannon introduced two fundamental properties for any block

cipher to have perfect secure, namely; diffusion and confusion. The

idea of diffusion is to hide the relationship between the cipher text

and plain text. Diffusion implies that each symbol (character or

byte or bit) in the cipher text is dependent on same or all symbols

in the plain text i.e., if a single symbol in the plain text is changed

several or all symbols in the cipher text will be changed. The idea

of confusion is to hide the relation between the cipher text and the

key. This will frustrate the adversary who tries to use the cipher

text to find the key i.e., if a single bit in the key is changed, most

or all bits in the cipher text will also be changed.

The diffusion effect can be introduced on cipher text by per-

mutation. The confusion effect can be introduced on cipher text

by substitution box or S-box. Most of the modern block ciphers

invariably use the S-box in different forms. In this thesis, the con-

struction of S-box and Inverse S-box used in AES algorithm with

necessary mathematical background are discussed in detail. The

input to an S-box could be an n-bit word, but the output can be an

m-bit or n-bit word, where the mapping from the inputs to the out

puts is predefined. S-boxes are an important component of sym-

metric cryptosystems. Because AES has only one standard S-box,

it has made it a target of algebraic attacks like the XSL (eXtended

22

Sparse Linearization) attack. Though none of these attacks have

succeeded, they provide an incentive for dynamic S-boxes.

1.3 Types of Attacks

Passive attacks: Passive attacks are in the nature of eavesdropping

on, or monitoring of, transmissions. Attacks threatening confiden-

tiality of information are snooping and traffic analysis. Snooping

refers to unauthorized access to an interception of data. For exam-

ple, a file inserted through the Internet may contain confidential

information. An unauthorized entity may intercept the transmis-

sion and use the contents for his/her own benefit. To prevent

snooping, the data can be made non illegible to the interceptor by

using enciphering technique.

Traffic analysis: Although the encipherment of data may make

it non intelligible for the interceptor, he/she can obtain some other

type of information by monitoring on line traffic. For example

he/she can find the electronic address of the sender and/or the

receiver.

Active attacks: Active attacks involve some modification of the

data stream or the creation of a false stream. The important active

attacks are:

• Attacks Threatening Integrity: The integrity of data can be

threatened by modification. After intercepting or accessing

23

the information, the attacker modifies the information to make

it beneficial to himself or herself or to others. Sometimes the

attacker simply deletes or destroys the message to harm the

system or to benefit from it.

• Masquerading: It happens when the attacker impersonates

somebody else. For example an attacker might steal the bank

card and PIN of a bank customer and pretend that he/she is

the customer.

• Replaying: The attacker obtains a copy of the message sent

by a user and later tries to replay/send it. For example, a

person sends a request to his bank to ask for payment to the

attacker, who had done a job for him. The attacker intercept

the message and sends it again to cause another payment from

the Bank.

• Repudiation: The sender of the message might later deny that

he/she has sent the message and the receiver of the message

might deny that he has received the message.

• Attacks threatening availability: It is a very common attack.

It may slow down or totally interrupt the service of a sys-

tem. The attacker can use several strategies to achieve this.

He might send so many bogus requests to a server that the

server crashes because of the heavy load. The attacker might

24

intercept and delete a server’s response to a client, making

the client to believe that the server is not responding. The

attacker may also intercept the requests from the clients, caus-

ing the clients to send the requests many times and over load

the system.

1.4 Cryptanalysis

The cryptanalytic attacks rely on the nature of the algorithm and

the general characteristics of the plain text or even some sample

of the plain text-cipher text pairs. This type of attack exploits the

characteristics of the algorithm to attempt to deduce a specific

plain text or to deduce the key being used.

1.4.1 Brute-force attack

It involves trying every possible key until an intelligible translation

of the cipher text into plain text is obtained. On an average, half

of all possible keys must be tried to achieve success. Table 1.3

shows the average time required for exhaustive search. With the

use of massively parallel organizations of microprocessors, it may

be possible to achieve processing rates many orders of magnitude

greater.

25

Table 1.3: The average time required for exhaustive search

Key

size(bits)

Number of al-

ternative keys

Time re-

quired at 1

decryption/µs

Time required

at 106 decryp-

tion/ µs

32 4.3× 109 35.8 minutes 2.15 milliseconds

56 7.2× 1016 1142 years 10.01 hours

128 3.4× 1038 5.4× 1024 years 5.4× 1018 years

168 3.7× 1050 5.9× 1036 years 5.9× 1030 years

1.4.2 Differential cryptanalysis

The rationale behind the differential cryptanalysis is to observe

the behavior of the pairs of the text blocks evolving along each

round of the cipher, instead of observing the evolution of a single

text block.

Consider the original plain text block m to consist of two halves

m0, m1. Each round of block cipher output is swapped. At each

round only one new m/2 bits block is created. Then the interme-

diate message halves are related as follows:

mi+1= mi−1 ⊕ f(mi, Ki) i = 1, 2, ..n

where Ki is round key and n is number of rounds.

To start with two messages m and m′, with known XOR differ-

ence ∆m= m⊕m′ and consider the difference between the inter-

mediate message halves ∆mi = mi ⊕m′i

∆mi+1 = mi+1 ⊕m′i+1

26

= [mi−1 ⊕ f(mi, Ki)⊕mi+1]

= [m′i−1 ⊕ f(m′i, Ki)]

Now, suppose that many pairs of inputs to f with the same differ-

ence yields the same output difference if the same sub key is used.

If we know mi−1 and mi with high probability, then we know mi+1

with high probability. If a number of such differences are deter-

mined, it is feasible to determine the sub key used in the function

f . It is found that in order to break a block cipher of 56-bit key

it needs 247 chosen plain text with 247 encryptions. Although 247

is certainly significantly less than 255 the need for the adversary

to find 247 chosen plain text makes this attack of only theoretical

interest.

1.4.3 Linear cryptanalysis

This attack is based on finding the linear approximations to de-

scribe the transformations performed in block ciphers. This method

can find a block cipher key given 243 known plain texts as compared

to 247 chosen plain text for differential cryptanalysis. Although

this is a minor improvement, because it may be easier to acquire

known plain text rather than chosen plain text, it still leaves linear

cryptanalysis infeasible attack on block ciphers.

27

1.5 Organization of the Thesis

This thesis addresses the enhancement of confidentiality and in-

tegrity using cryptographic techniques. The rest of the thesis has

been organised as follows: Chapter 2 deals with the literature sur-

vey and the problem statement of the work. Chapter 3 deals with

dynamic S-box generation and the avalanche criteria of the S-box.

The static S-box used in the present Advanced Encryption Stan-

dard satisfies only 64 percent avalanche criteria. It has been shown

that there are S-boxes which can satisfy the maximum avalanche

criteria. In Chapter 4, the present AES algorithm and the modi-

fied AES algorithm with the dynamic S-box have been presented.

In Chapter 5, the stream cipher generated based on the dynamic

S-box has been discussed. In Chapter 6, another important secu-

rity aspect integrity HASH function has been discussed. In this

chapter, the modified Whirlpool Hash function generation with

dynamic S-box is discussed and the results have been tabulated.

Chapter 7 concludes this thesis with the directions for the future

research.

28