Chapter 1
Introduction
In today’s information age, communication plays a very important role and has contributed heavily to the growth of technology.
The Electronic security has increasingly involved in making communication 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 protected 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 unintelligible to all but the intend to receive. On the other hand, the
1
Cryptanalysis is the science and art of solving cryptosystems to recover 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 assurance that the data received are exactly as sent by an authorized
entity. The authentication is the assurance that the communication entity is the one that it claims to be. The present day Cryptography 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 nonstandard 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 carefully 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 possessions. Scribes could easily make these records by drawing a picture
of a cow or a boat followed by a number. But as the language be2
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 substi3
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 obscured 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 actually 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 ’Skytale’ (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 during 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 description 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 substitution and polygraphic substitution schemes. The frequency analysis
was based on the fact that in any given stretch of a language, letters 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 turning 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 deciphering 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 system 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 polyalphabetic, meaning that instead of there being a one-to-one relationship
between each letter and its substitute, there is a one-to-many relationship 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, electro statically generated signals which stood for letters of the alphabet 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 punctuation 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
..
.
...
..
.
D
..
.
Y
Y
Z
A
B
C
D
E
F
... W X
Z
Z
B
C
D
E
F
G
H
... X
9
C
..
.
Y
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 between 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 dividing 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 alliance 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 talkers 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
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
I
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 Government.
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 algorithms 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 computer 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 designed to protect data and to thwart hackers is computer security.
In symmetric encipherment or secret key cryptography, an entity A can send a message to another entity B, over an insecure
channel with the assumption that an adversary X cannot understand 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 depending 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 output. 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 algorithm 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 ≡ A0 × C + B 0 mod N , where A and B are
encryption Keys A’ = A−1 and B’ = −A−1 B are decryption
keys.
• Mono Alphabetic Substitution Cipher: In this method a mapping is created between each plain text character and the corresponding 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 cipher 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 predetermined order.
• Keyless Transposition cipher: There are two methods in Keyless 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 table 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 cryptanalysts 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 National Institute of Standards and Technology (NIST)), as Federal
Information Processing Standard 46 (FIPS PUB 46) The parameters 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 possible 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 algorithm, 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 requirement for different implementations such as hardware,
software or smart cards.
• Implementation: The algorithm must have flexibility and simplicity that is implementation must be possible on any platforms.
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 permutation. 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 construction 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 symmetric 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 confidentiality of information are snooping and traffic analysis. Snooping
refers to unauthorized access to an interception of data. For example, a file inserted through the Internet may contain confidential
information. An unauthorized entity may intercept the transmission 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 system. 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, causing 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
Number of al- Time
size(bits)
ternative keys
quired
re- Time required
at
1
at 106 decryp-
decryption/µs
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 intermediate 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 m0 , with known XOR difference ∆m= m ⊕ m0 and consider the difference between the intermediate message halves ∆mi = mi ⊕ m0i
∆mi+1 = mi+1 ⊕ m0i+1
26
= [mi−1 ⊕ f (mi , Ki ) ⊕ mi+1 ]
= [m0i−1 ⊕ f (m0i , Ki )]
Now, suppose that many pairs of inputs to f with the same difference 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 determined, 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 describe 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 integrity using cryptographic techniques. The rest of the thesis has
been organised as follows: Chapter 2 deals with the literature survey 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 Standard 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 modified 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 security 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