cryptography - The Direct Data

daughterinsectAI and Robotics

Nov 21, 2013 (5 years and 4 months ago)









The requirement of
information security

within an organization has under gone
two major changes in the last several decades. Before the widespread use of data
processing equipment, the security of

information felt to be valuable to an organization was
provided primarily by physical and administrative means. An example of the former is the
use of rugged filing cabinets with a combination lock for storing sensitive documents. An
example of the latter

is personnel screening procedures used during the hiring process.

With the introduction of 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

system, such as a time
sharing system, and the need is even more acute for system
that can be accessed over public telephone network, data network, or the Internet. The
generic name for the collection of the tools designed to protect data and to thwart ha
ckers is
computer security.

The second major change that affected security is the introduction of distributed system and
the use of network and communication facilities for carrying data between terminal user
and computer and between computer and comput
er. Network security measure are needed
to protect data during their transmission. In fact, the term
network security
is somewhat

because virtually all business, government, and academic organization
interconnect their data processing equipment

with a collection of interconnected networks.
Such a collection is often referred to as an internet, and the term
internet security
is used.

There are no clear boundaries between these two forms of security. For example, one of the
most publicize
d types of attack on information system is the computer virus. A virus may
be introduced into a system physically when it arrives on a diskette and is subsequently
loaded onto a computer. Viruses may also arrive over an internet. In either case, once the
irus is resident on a computer security tools are needed to detect and recover from the


is the study of mathematical techniques related to aspects of
information security, such as confidentially or privacy ,data integrity and entity
uthentication. Cryptography is not only means of providing information security, but
rather one set of techniques. Confidentially means keeping information secret from all but
those who authorized to see it. Data integrity means ensuring information has no
t been
altered by unauthorized or unknown means. Entity authentication means corroboration of
the identify of an entity.



There are some

of cryptographic algorithm. They are level security,
performance , and ease of implementation. Level
security defined by an upper bound on the
among of work necessary to defeat the objective. Performance refers to the efficiency of an
algorithm in a particular mode of an operation. Ease of implementation refers to the
difficulty of realizing the algorithm

in practical implementation.

There are several
of security. They are security service, security mechanism, and
security attack. Security service means a service that enhances the security of the data
processing system and information transfers

of an organization. Security mechanism
means that is designed to detect, prevent, or recover from a security attacks. Security attack
means any action that compromises the security of information owned by an organization.


means the pr
ocess of converting from plaintext to ciphertext. A key is a piece
of information , usually a number that allows a receiver. Another key also allows a receiver
to decode messages sent to him or her. There are some types of encryption. They are
classical te
chniques, modern techniques, and public
key encryption. In Classical techniques
there are substitution techniques and transposition techniques. In substitution techniques
there are Caesar cipher, monoalphabetic cipher and polyalphabetic cipher. In Modern
echniques there are block cipher , stream cipher and DES algorithm. In Public
encryption the RSA algorithm is there.

Cryptography has provided us with
Digital Signatures

that resemble in
functionality the hand
written signature and
Digital Certifica

that related to an ID
or some other official documents. There are some

of cryptography. They are
secure communication, identification, secret sharing, electronic commerce, key recovery
and remote access.

Modern cryptography provid
es essential techniques for securing information and
protecting data.









Definition of




Categories of cryptographic algorithm



Related Terms of cryptography



Goals of cryptography



Characteristics of cryptography



Aspects of Security



The OSI security Architecture



Model For Network Security



Simplified Model Of Conventional Encryption



Classical Encryption Technique



Substitution Technique


Technique Transposition


Modern Technique



Stream & Block cipher


ffusion & Confusion


DES Algorithm


Key Encryption



Principle Of Public
Key Cryptography


Key cryptosystem


Key cryptosystem : Secrecy


Key cryptosystem : Authentication


Key cryptosystem : Secrecy & Authenticat


RSA Algorithm


Advantages & Benefits



ClassicSys as a standard


Advantages & Benefits For END


Advantages & Benefits For Authority…


Technical Advantages & Benefits…


Comparison between DES, RSA, & SED Algorithm 30



Of Cryptography








Due to the rapid growth of digital communication and electronic data exchange information
security has become a crucial issue in industry, business a
nd administration. Assume a
sender referred to here and in what follows as Alice (is commonly used) wants to send a
message m to a receiver referred to as Bob. She uses an insecure communication channel.
For example, the channel could be a computer network

or a telephone line. There is a
problem if the message contains confidential information. The message could be
intercepted and read by eavesdropper. Or even worse, some might be able to modify the
message during transmission, so Bob does not detect the ma

Cryptography has provided us with
digital signature
that resemble in functionality
the hand
written signature and
digital certificates

that related to an ID CARD or other
official documents. Modern cryptography provides essential techniques f
or securing
information and protecting data.

Definition of cryptography

Cryptography is the study of mathematical techniques related to aspects of
information security, such as confidentially or privacy, data integrity and entity
authentication. Crypto
graphy is not the only means of providing information security, but
rather one set of techniques.

Categories of cryptographic algorithm

There are main two types of cryptographic algorithm.


Symmetric key


Asymmetric key

Symmetric key

Sender an
d Receiver share
a key.

A secret piece of information used to encrypt or decrypt the message.

If a key is secret, than nobody other than sender or receiver can read


If Alice and bank each has secret key, than they may se
nd each other

private message.

The task of privately choosing a key before communication, however

be problematic.



Asymmetric key

Solves the key exchange problem by defining an algorithm which uses


each of which can be use to encrypt the message.

If one is used to encrypt a message, another key must be used to

decrypt it.

This makes it possible to receive secure message by simply publishing

key (public key) and keeping anoth
er secret (private key).

Any one may encrypt a message using public key, but only the owner

the public key is able to read it.

In this way Alice may send private message to owner of a key
pair (the

bank) by encrypting it using their
key. Only bank can decrypt it.

Related Terms


An original intelligible message or data that is fed into the algorithm as

Cipher text:


coded message is known as Cipher text. That is depends on plaintext
and secret key.


The process of converting from plaintext to cipher text that is known as


Restoring the plaintext from cipher text that is known as Decryption.


The many schemes used for enciphering consti
tute the area of study
known as Cryptography. Such as a scheme is known as Cryptographic system or Cipher.


Techniques used for deciphering a message without any knowledge of
enciphering details fall into the area of Cryptanalysis.


ptanalysis is what the layperson calls 'Breaking The Code '.


The areas of cryptography and cryptanalysis together are called

Goals of cryptography



The main goals of cryptography are


Confidentially or privacy


Data i





1) Confidentially or Privacy

Keeping information secret from all, but those who are authorized to see it.
Confidentially is the protection of transmitted data from passive attacks. With respect to t
content of data transmission, several levels of protection can be identified. The broadest
service protects all user data transmitted between two users over a period of time.

The aspect of Confidentially is the protection of traffic flow from analysi
s. This
requires that an attacker not be able to observe to source and destination, frequency, length
or any other characteristics of the traffic on a communication facility.

2) Data Integrity:

Ensuring the information has not been altered by unauthor
ized or unknown means.
One must have the ability to detect data manipulation by unauthorized parties. Data
manipulation includes such things as insertion, deletion, and substitution

3) Authentication:

Corroboration of the identify of an entity. Authe
ntication is a service related to
identification. This function applies to both entities and information.

4) Non

repudiation prevents either sender or receiver from denying a message. Thus,
when a message is sent, the receiver

can prove that the message was in fact send by the
alleged sender. Similarly, when a message is received, the sender can prove the alleged
receiver in fact received that message.

Characteristics of a cryptographic algorithm

The main characteristics of
cryptographic algorithm are


Level of security




Ease of implementation


Level Of Security:



Typically the level of security is defined by an upper bound on the among of work
necessary to defeat the objective. This is som
etimes called the 'Work Factor'.

Work Factor could be defined as the minimum amount of work required to compete
the private key when given the public key, or in the case of the symmetric key scheme to
determine the secret key.

A functionality algorithm
will need to be combined to meet various information
security objectives. Which algorithm is most effective for the given objective, will be d

determined by the basic properties of the algorithm.

The methods of operations algorithm when applied in variou
s ways and with various inputs
will typically exhibit different characteristics. Thus, one algorithm could provide very
different functionality depending on its mode of operation or usage.

2) Performance :

Performance refers to the efficiency

of an algorithm in a particular mode of operation . For
example, the number of bits/sec at which it can encrypt may rate an encryption algorithm.

3) Ease Of Implementation


This refers to the difficulty of realizing the algorithm in a practi
cal instantiation, and might
include the complexity of implementing in an either software or a hardware environment.

The relative importance of various criteria depends to a large extent on the
application and resources available. For example, in an envir
onment where computing
power is limited , one may have to trade off very high level of security for better system

Aspects Of Security

To assess the security needs, of an organization effectively and choose
various security products a
nd policies, the manager responsible for security needs some
systematic way of defining the requirements for security and characterizing the approaches
to satisfied those requirements. One approach is to consider three aspects of information


rity attack


Security mechanism


Security service


Security Attack:

Any action that compromises the security of information owned by an organization.






A mechanism that is designed to detect, prevent or recover from a security at


Security Services:

A service that enhances the security of the data processing system and the information
transfers of an organization. The services are intended to counter security attacks, and they
make use of one or more security mechanism

to provide the service.

The OSI Security Architecture

To assess the security needs, of an organization effectively and choose various
security products and policies, the manager responsible for security needs some systematic
way of defining the requir
ements for security and characterizing the approaches to satisfied
those requirements. This is difficult enough in a centralized data
processing environment;
with the use of local area and wide area network, the problems are compounded.

T (The Interna
tional Telecommunication Union (ITU) Telecommunication
Standardization Sector (ITU
T) United Nation (UN)
sponsored agency that develops
standard, called Recommendations, relating to telecommunication and to Open System
Interconnection (OSI)) Recommendatio
ns X.800,
security Architecture

for OSI, defines
such a systematic approach. The OSI security architecture is useful to managers as way of
organization the task of providing security. Further more, because this architecture was
developed as international s
tandards, computer and communications vendors have
developed security feature for their products and services that relate to this structured
definition of services and mechanisms.

Security Services:

X.800 defines a security service as a servic
e provided by a protocol layer of
communication open system, which ensures adequate security of the system or of data

X.800 divides these services into five categories and fourteen specific services.




Access Control


Data confident
ially or Privacy


Data integrity








Corroboration of the identity of an entity. Two specific authentication services are
defined in the standard.

Peer Entity Authentication:

Used in association with a logical connec
tion to provide confidence in the identity of the
entities connected.

Data Origin Authentication:

In connection less transfer, provides assurance that the source of received data is as


Access Control:

In the context of network security, access control is the ability to limit and control the
access to host system and application

via communication links. To achieve this, each entity
trying to gain access must first be identified, or authenticated, so that access rights can be
tailored to the individual.


Data Confidentially Or Privacy:

The protection of data from unauthorized
disclosure. Four specific services of
confidentially are

Connection Confidentially:

The protection of all user data on a connection.

Connectionless Confidentially:

The protection of all user data in a single data book.

Selective Field Confidentia

The confidentially of selected fields within the user data on a connection or in a single data


flow confidentiality:

The protection of information that might be derived from observation of traffic flow.


Data Integrity:



The assurance that data received is exactly as sent by an authorized entity. That means no
modification insertion, deletion or replay. There are five types of specific services.

Connection Integrity With Recovery:

Provides for the integrity of all use
r data on a connection and detects any modification,
insertion, deletion or reply
of any data within an entries data sequence, with recovery

Connection Integrity Without Recovery:

As above, but provides only detection without recovery.

Field Connection Integrity:

Provides for the integrity of selected fields within the user data of a data block transferred
over a connection and takes the form of determination of whether the selected fields have
been modified, inserted, deleted
or replayed.

Connectionless Integrity:

Provides for the integrity of a single connectionless data block and may take the form of
detection of data modification. Additionally, a limited form of replay detection may be

Field Connecti
onless Integrity:

Provides for the integrity of selected fields within a single connectionless data block; takes
the form of determination of a whether the selected field have been modified.



Provides protection against denial by on
e of the entities involved in a
communication of having participated in all or part of the communication. There are two
types of specific services in Non

repudiation, origin:

Proof that the specific parties sent the massage.

epudiation, Destination:

Proofs that the massage was receive by the specific parties.



Security mechanism:

As can be seen the mechanism are divided into those that are implemented in a specific
protocol layer and those that are not specific to any
particular protocol layer or security
service. X.800 distinguishes between reversible encipherment mechanism is simply an
encryption algorithm that allows the data to be encrypted and subsequently decrypted.
Irreversible encipherment mechanism includes has
h algorithm and used in digital signature
and message authentication application.

Security Attacks:

A useful means of classifying security attacks, used in x.800, is in term of
passive attacks

active attacks.
A passive attack attempts to learn

or make use of information from the
system but does not affect system resources. An active attack attempts to alter system
resources or affect their operation.

Passive Attacks:

Passive attacks are in the nature of eavesdropping on, or monitoring of
, transmissions. The
goal of the opponent is to obtain information that is being transmitted. Two types of
passive attacks are release of message contents and traffic analysis.

release of message

contents is easily understood. A telephone conversatio
n, an
electronic mail message, and transferred file may contain sensitive or confidential
information. We would like to prevent the opponent from learning the contents of these

A second type of passive attacks,
traffic analysis
, is subtle
r. Suppose that we had a
way of masking the contents of messages or other information traffic so that opponents,
even if they captured the message, could not extract the information from the message. The
common technique of masking contents is encryption.
If we had encryption protection in
place, an opponent might still be able to obverse the pattern of these messages. The
opponent could determine the location and identity of communicating hosts and could
observe the frequency and length of messages being e
xchanged. This information might be
useful in guessing the nature of the communication that was taking place.

Passive attacks are very difficult to detect because they do not involve any
alteration of the data. How ever, it is feasible to prevent the su
ccess of these attacks,
usually by means of encryption. Thus, the emphasis in dealing with passive attacks is on
prevention rather then detection.

Active Attacks:

Active attacks involve some modification of the data stream or the creation of a
false s
tream and can be subdivided into four categories: masquerade, replay modification of
messages, and denial of service.




takes place when one entity pretends to be a different entity. A
masquerade attack usually includes one of the other forms
of active attack.

involves the passive capture of a data unit and it's subsequent
retransmission to produce an unauthorized effect.

Modification of messages

simply means that some portion of a legitimate message
is altered, or that messages are
delayed or reordered to produce an unauthorized effect. For
example, a message meaning "Allow John Smith to read confidential file accounts" is
modified to mean "Allow Fred Brown to read confidential file accounts".

denial of service

prevents or inh
ibits the normal use or management of
communication facilities. This attack may have a special target; for example an entity may
suppress all messages directed to particular destination. Another form service denial is the
disruption of an entire network, e
ither by disabling the network or by overloading it with
messages so as to degrade performance.

Active attacks present the opposite characteristics of passive attack where as
passive attacks are difficult to detect, measures are available to prevent thei
r success. On
other hand it is quit difficult to prevent active attacks absolutely, because to do so would
require physical protection of all communications facilities and paths at all times. Instead,
the goal is to detect than to recover from any disrupti
on or delays caused by them. Because
the detection as a deterrent effect, it may also contribute to prevention.

A Model For Network Security:

A model for much of what we will be discussing is captured, in very general terms,
in figure. A message is
to be transferred from one party to another across some sort of
Internet. The two parties, who are the principals in this transaction, must cooperate for the
exchange to take place. A logical information channel is established by defining a route
through t
he Internet from source to destination and by the cooperative use of
communication protocol (e.g., TCP/IP) by the two principles.

Security aspects come in to play when it is necessary or desirable to protect the
information transmission from an opponent
who may present a threat to confidentiality,
authenticity, and so on. All the techniques for providing security have to components:

A security
related transformation on the information to be sent. Examples
include the encryption of the message, which scra
mbles the message so that it is unreadable
by the opponent, and the addition of a code based on the contents of the message, which
can be used to verify the identity of the sender.



Model for Network Security

Some secret info
rmation shared by the two principals and, it is hoped,
unknown to the opponent. An example is an encryption key used in conjunction with the
transformation to scramble the message before transmission and unscramble it on

A trusted third part
y may be needed to achieve secure transmission. For example, a third
party may be responsible for distributing the secret information to the two principals while
keeping it from any opponent. Or a third party may be needed to arbitrate disputes between

two principals concerning the authenticity of a message transmission.

This general model shows that there are four basic tasks in designing a particular
security service:

Design an algorithm for performing the security
related transformation. The

algorithm should be such that an opponent cannot defeat its purpose.

Generate the secret information to be used with the algorithm

Develop methods for the distribution and sharing of the secret information.

Specify of protocol to be used by the two princi
pals that makes use of the
security algorithm and secret information to achieve a particular security service.

However, there are other security related situations of interest that do not neatly fit this
model but that are considered here. A general model
of this other situation illustrated by
figure, which reflects concern for protecting an information system from unwanted access.
Most readers are familiar with the concerns caused by the existence of hackers, who
attempt to penetrate systems that can be ac
cessed over a network. The hacker can be some
one who, with no malign intent, simply get satisfaction from breaking and entering a
computer system. Or, the intruder can be a disgruntled employee who wishes to do damage,
or a criminal who seeks to exploit c
omputer assets for financial gain (e.g., obtaining credit
card numbers or performing illegal money transfers)

Another type of unwanted access is the placement in a computer system of logic that
exploits vulnerabilities in the system and that can affect a
pplication program as well as
utility programs such as editor and compilers. Programs can present two kinds of threats:



Information access threats intercept or modify data on behalf of users who
should not have access to that data.

Service threats exploi
t services flaws in computers to inhibit use by legitimate

Network Access Security Model

Viruses and worms are two examples of software attacks. Such attacks can be
introduced into a system by means of a disk that co
ntain unwanted logic concealed in
otherwise useful software.

The security mechanism needed to coped with unwanted access fall into two broad
categories. The first categories might be termed a gatekeeper function. It includes
based login procedu
res that are designed to deny access to all but authorized user
and screening logic that is designed to detect and reject worms, viruses, and other similar
attacks. Once is gained, by either an unwanted users or unwanted software, the second line
of defens
e consists of a variety of internal controls that monitor activity and analyze stored
information in an attempt to detect the presence of unwanted intruders.

Simplified Model of Conventional Encryption:

There are two requirements for secure use of
conventional encryption:

We need a strong encryption algorithm. At s minimum, we would like the
algorithm to be such that an opponent who knows the algorithm and has access to one or
more cipher text would be unable to decipher the cipher text or figure o
ut the key. This
requirement is usually stated in a stronger form : The opponent should be unable to decrypt
cipher text or discover the key even if he or she is in possession of a number of cipher texts
together with the plaintext that produced each ciph
er text.

Sender and receiver must have obtained copies of the secret key in a secure
fashion and must keep the key secure. If some one can discover the key and knows the
algorithm, all communication using this key is readable.

We assume that it is imprac
tical to decrypt a message on the basis of the cipher text

plus knowledge of the encryption/decryption algorithm. In other words we do not



need to keep the algorithm secret; we need to keep only the key secret.

This feature of symmetric encryption is wha
t makes it feasible for widespread use. The fact
that the algorithm need not be kept secret means that manufacturers can end has developed
cost chip implementations of data encryption algorithms. These chips are widely
available and incorporated into a

number of products. With the use of symmetric
encryption, the principal security problem is maintaining the secrecy of the key.

Simplified Model of Conventional Encryption



Cryptographic systems are characterized along three independent dimensions.

The type of operations used for transforming plain text to cipher text.

encryption algorithms are based on two general principles: substitution, in which each
element in
the plaintext (bit, letter, group of bits or letters) is mapped in to another element,
and transposition, in which elements in the plaintext are rearranged. The fundamental
requirement is that no information be lost. Most systems, referred to as product sy
involve multiple stages of substitutions and transpositions.

The number of keys used.

If both sender and receiver use the same key, the
system is referred to as symmetric, single
key, secret
key, or conventional encryption. If
the sender and recei
ver each use a different key, the system is referred to as asymmetric,
key, or public
key encryption.









Secret key shared by
sender & receiver.

Secret key shared by
sender & receiver.



The way in which the plaintext is processed.

A block cipher processes the
input one block of elements at a time, producing an output block for each i
nput block. A
stream cipher processed the input elements continuously, producing output one element at a
time, as it goes along.


There are two general approaches to attacking a conventional encryption scheme:


nalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of
the general characteristics of the plaintext or even some sample plaintext
cipher text pairs.
This type of attack exploits the characteristics of the algorithm to attempt to

deduce a
specific plaintext or to deduce the key being used. If the attack succeeds in deducing the
key, the effect is catastrophic: All future and past messages encrypted with that key are

force attack:

The attacker tries every pos
sible key on a piece of cipher text until an intelligible
translation into plaintext is obtained. On average, half of all possible keys must be tried to
achieve success.

Classical Encryption Techniques:

A study of these techniques unable us to illust
rate the basic approaches to symmetric
encryption used today and the types of cryptanalytic that must be anticipated.

The two basic building blocks of all encryption techniques are substitution and
transposition. We examine these in the next two sections
. Finally, we discuss a system that
combines both substitution and transposition.

Substitution Techniques:

A substitution technique is one in which the letters of plaintext are replaced by
other letters or by numbers or symbols. If the plaintext is v
iewed as a sequence of bits, then
substitution involves replacing plaintext bit patterns with cipher text bit patterns.

Caesar Cipher:

The earliest known use of a substitution cipher, and the simplest, was by Julius
Caesar. The Caesar cipher involve
s replacing each letter of the alphabet with the letter
standing three places further down the alphabet. For example



Plain: meet me after the toga party


Note that the alphabet is wrapped around, so that the latter
following Z is

A. We can define the transformation by listing all possibilities, as follow:

Plain: a b c d e f g h I j k l m n o p q r s t u v w x y z

Cipher: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

Let us assign a numeric equivalent to

each letter:

Then the algorithm can be expressed as follows. For each plaintext letter
p, s
ubstitute the
cipher letter

C = E

) = (P+3) mod (26)

A shift may be of any amount, so that the general Caesar algorithm is

C = E

) = (
) mod (26)

takes on a value in the range 1 to 25. The decryption algorithm is simply


= D(C) = (C

mod (26)

Transposition Techniques:

All the techniques examined so far involve the substitution of

a cipher text
symbol for a plaintext symbol. A very different kind of mapping is achieved by performing
some sort of permutation on the plaintext letters. This technique is referred to as a
transposition cipher.

The simplest such cipher is the rail fen
ce technique, in which the plaintext is
written down as a sequence of diagonals and then read off as a sequence of rows. For,
example, to encipher the message " meet me after the toga party " with a rail fence of depth
2, we write the following.

M e m
a t r h t g p r y

E t e f e t e o a a t

The encrypted message is


This sort of thing would be trivial to crypt analyze. A more complex scheme is to write the
messages in a rectangle, row by row, and read the message of
f, column by column, but
permute the order of the columns. The order of the columns then becomes, the key to the
algorithm. For example,

Key: 4 3 1 2 5 6 7

Plaintext: a t t a c k p

o s t p o n e

d u n t I l t

w o a m x y z

Cipher text:




A pure transposition cipher is easily recognized because it has the same letter frequencies
as the original plaintext. For the type of columnar tr
ansposition just shown, cryptanalysis is
fairly straightforward and involves laying out the cipher text in a matrix and playing around
with column positions. Digram and trigram frequency tables can be useful.

The transposition cipher can be made signific
antly more secure by performing more
than one stage of transposition. The result is a more complex permutation that is not easily
reconstructed. Thus, if the foregoing message is re
encrypted using the algorithm.


4 3 1 2 5 6 7


t t n a a p t

m t s u o a o

d w c o I x k

n l y p e t z


Modern Techniques:

Virtually all
symmetric block encryption algorithm in

current use is based
on a structure referred to as a Feistel block cipher. We begin with a comparison of stream
ciphers and block ciphers.

Stream ciphers:

A stream cipher is one that encrypts a digital data stream one bit or one byte at a time.
ample of classical stream ciphers is auto keyed Vigenere cipher and the Vernam cipher.

Block ciphers:

A block cipher is one in which a block of plaintext is treated as a whole and used to
produced a cipher text block of equal length. Typically, a bloc
k size of 64 or 128 bits is
used. Using some of the modes of operation explained later in this chapter, a block cipher
can be used to achieve the same effect as a stream cipher. Far more effort has gone into
analyzing block ciphers. In general, they seem
applicable to a broader range of applications
than stream ciphers. The vast majority of network
based symmetric cryptographic
applications make use of block ciphers.

Diffusion and Confusion:

The terms diffusion and confusion were introduced by Claud
e Shannon to capture
the two basic building blocks for any cryptographic system. Shannon's concern was to
thwart cryptanalysis based on statistical analysis. The reasoning is as follows. Assume the


attacker has some knowledge of the statistical characteris
tics of the plaintext. For example,
in a human
readable message in some language, the frequency distribution of the various
letters may be known. Or there may be words or phrases likely to appear in the message. If
these statistics are in any way reflecte
d in the cipher text, the cryptanalyst may be able to
deduce the encryption key, or part of the key, or at least a set of keys likely to contain the
exact key.

Other than recourse to ideal systems, Shannon suggests two methods for frustrating
l cryptanalysis: diffusion and confusion. In

the statistical structure of the
plaintext is dissipated into long
range statistics of the cipher text. This is achieved by
having each plaintext digit affect the value of many cipher text digits, w
hich is equivalent
to saying that ciphertext digit is affected by many plaintext digits. An example of diffusion
is to encrypt a message M = m1, m2, m3,… of characters with an averaging operation :


Yn =

n + i

(mod 26)


Adding k successive letters to get a ciphertext letter Yn. One can show that the statistical
structure of the plaintext has been dissipated. Thus the letter frequencies in the ciphertext
will be more nearly equal than in the plaintex
t; the Digram frequencies will also be more
nearly equal, and so on. In a binary block cipher, diffusion can be achieved by repeatedly
performing some permutation of the sata followed by applying a function to that
permutation; the effect is that bits from

different positions in the original plaintext
contribute to a single bit of ciphertext.

Every block cipher involves a transformation of a block of plaintext into a block of
ciphertext, where the transformation depends on the key. The mechanism of diffus
ion seeks
to make the statistical relationship between the plaintext and ciphertext as complex as
possible in order to thwart attempts to deduce that key. On the other hand,

to make the relationship between the statistics of the ciphertext
and the value of the
encryption key as complex as possible, again to thwart attempts to discover the key. Thus,
even if the attacker can get some handle on the statistics of the ciphertext, where the
transformation depends on the key. The mechanism of diff
usion seeks to make the
statistical relationship between the plaintext and ciphertext as complex as possible in order
to thwart attempts to deduce that key. On the other hand, confusion seeks to make the
relationship between the statistics of the ciphertex
t and the value of the encryption key as
complex as possible, again to thwart attempts to discover the key. Thus, even if the attacker
can get some handle on the statistics of this, as Federal Information Processing Standards
46 (FIPS pub 46). The algorith
m itself is referred to as the Data Encryption Algorithm
(DEA). For EDS, data are encrypted in 640bit blocks using a 56
bit key. The algorithm
transforms 64
bit input in a series of steps into a 64
bit output. The same steps, with the
same key, are used
to reverse the encryption.

The DES enjoys widespread use. It has also been the subject of much controversy
concerning how secure the DES is,. To appreciate the nature of the controversy, let us
quickly review the history of the DES.



In the late 1960s,
IBM set up a research project in computer cryptography led by
Horst Feistel. The project concluded in 1971 with the development of an algorithm with the
designation LUCIFER (FEIS73), which was sold to Lloyd's of London for use in a cash
dispensing system,
also developed by IBM LUCIFER is a Feistel block cipher that operates
on blocks of 64 bits, using a key also of 128 bits. Because of the promising results
produced by the LUCIFER project, IBM embarked on an effort to develop a marketable
commercial encrypt
ion product that ideally could be implemented on a single chip. The
effort was headed by Walter Tuchman and Cart Meyer, and if involved not only IBM
researchers but also out
side consultants and technical advice from NSA. The outcome of
this effort was a r
efined version of LUCIFER that was more resistant to cryptanalysis but
that had a reduced key size of 56 bits, to fit on a single chip.

In 1973, the National Bureau of Standards (NBS) issued a request for proposals for
a national cipher standard. IBM s
ubmitted the results of its Tuchman
Meyer project. This
was by far the best algorithm proposed and was adopted in 1977 as the Data Encryption

Before its adoption as a standard, the proposed DES was subjected to intense
criticism, which has not
subsided to this day. Two areas drew the critics’ fire. First, the key
length in IBM's original LUCIFER algorithm was 128 bits, but that of the proposed system
was only 56 bits, an enormous reduction in key size of 72 bits. Critics feared that this

length was too short to withstand brute
force attacks. The second area of concern was
that the design criteria for the internal structure of DES, the S
boxes, were classified. Thus,
users could not be sure that the internal structure of DES was free of an
y hidden weak
points that would enable NSA to decipher messages without benefit of the key. Subsequent
events, particularly the recent work on differential cryptanalysis, seem to indicate that DES
has a very strong internal structure. Furthermore, accordin
g to IBM participants, the only
changes that were made to the proposal were changed to the S
boxes, suggested by NSA,
that removed vulnerabilities identified in the course of the evaluation process.

Whatever the merits of the case, DES has flourished and

is widely used, especially
in financial applications. In 1994, NIST reaffirmed DES for federal use for another five
years; NIST recommended the use of DES for applications other than the protection of
classified information. In 1999, NIST issued a new ver
sion of its standard that indicated
that DES should only be used for legacy systems and that triple DES (which in essence
involves repeating the DES algorithm three times on the on plaintext using two or three
different keys to produce the ciphertext) be u

DES Encryption:



The overall scheme for DES encryption is illustrated in figure. As with any encryption
scheme, there are two inputs to the encryption function: the plaintext to be encrypted and
the key. In this case, the plaintext must be 64 bi
ts in length and the key is 56 bits in length.

General Depiction of DES Encryption Algorithm

Looking at the left
hand side of the figure, we can see that the processing of the plaintext
proceeds in three phases. First, the

bit plaintext passes through an initial permutation
(IP) that rearranges the bits to produce the
permuted input.
This is followed by a phase
consisting of 16 rounds of the same function, which involves both permutation and
substitution functions. The o
utput of the last (16) round consists of 64 bits that are a
function of the input plaintext and the key. The left and right halves of the output are
swapped to produce the
Finally, the preoutput is passed through a permutation


that is the invers
e of the initial permutation function, to produce the 64
bit ciphertext. With
the exception of the initial and final permutation, DES has the exact structure of a Feistel

The right
hand portion of figure shown the way in which the 56
bit key is u
Initially, the key is passed through a permutation function. Then, for each of the 16 rounds,
subkey (Ki)
is produced by the combination of a left circular shift and a permutation. The
permutation function is the same for each round, but a different

subkey is produced because
of the repeated iteration of the key bits.

key cryptography:

The development of public
key cryptography is the greatest and perhaps the only
true revolution in the entire history of cryptography. From its

earliest beginning to modern
times, virtually all cryptographic system have been based on the elementary tools of
substitution and permutation.

Principle of Public
key cryptosystem:

The concept of public
key cryptography evolved from an attempt to a
ttack two of
the most difficult problems associated with symmetric encryption. The first problem is that
of key distribution.

As we have seen, key distribution under symmetric encryption requires either

That to communicants already share a key, which so
me how has been
distributed to them; or

The use of a key distribution center Whitfield Diffie. One of the discoverers
of public
key encryption (along with Martin Hellman, both at Stanford University at the
time), reasoned that this second requirement nega
ted the very essence of cryptography, the
ability to maintain total secrecy over your own communication. As Diffie put to (DIFF88),
" what good would it do after all to develop impenetrable cryptosystems, if their users were
forced to share their keys with

a KDC that could be compromised by either burglary or
subpoena? "

The second problem that Diffie pondered, and one that was apparently unrelated to the first
was that of " digital signatures ". If the use of cryptography was to become widespread, not
st in military situations but for commercial and private purposes, then electronic message
and documents would need the equivalent of signatures used in paper documents. That is,
could a method be devised that would stipulate, to the satisfaction of all pa
rties that a
digital message had been sent by a particular person? This is a somewhat broader
requirement than that of authentication, and its characteristics and ramifications are

In the next subsection, we look at the overall framework for publ
key cryptography. Then
we examine the requirements for the encryption/decryption algorithm that is at the heart of
the scheme.



key cryptosystems:

The public
key algorithms rely on one key for encryption and a different but related
key for de
cryption. These algorithms have the following important characteristics:

It is computationally infeasible to determine the decryption key given only
knowledge of the cryptographic algorithm and the encryption key.

In addition, some algorithms, such as R
SA, also exhibit the following characteristics:

Either of the two related keys can be used for encryption , with other used
for decryption.

A public
key encryption scheme has six ingredients.


This is the readable message or data that

is fed into the
algorithm as input.

Encryption algorithm:

The encryption algorithm performs various
transformations on the plaintext.

Public and private key:

This is a pair of keys that have been selected so
that if one is used for encryption, the ot
her is used for decryption. The exact
transformations performed by the encryption algorithm depend on the public or private key
that is provided as input.


This is the scrambled message produced as input. It depends
on the plaintext and the ke
y. For a given message, two different keys will produce two
different ciphertexts.

Decryption algorithm:

This algorithm accepts the ciphertext and the
matching key and produces the original plaintext.

The essential steps are the following:

Each user
generates a pair of keys to be used for the encryption and
decryption of messages.

Each user places one of the two keys in a public register or other accessible
file. This is the public key. The companion key is kept private. As figure suggests, each

maintains a collection of public keys obtained from others.

If Bob wishes to send a confidential message to Alice, Bob encrypts the
message using Alice's public key.

When Alice receives the message, she decrypts it using her private key. No
other recipie
nt can decrypt the message because only Alice knows Alice's private key.

With this approach, all participants have access to public keys, and private keys, are
generated locally by each participant and therefore need never be distributed. As long as a
tem controls its private key, its incoming communication is secure. At any time, a
system can change its private key and publish the companion public key to replace its old
public key.



Table shows some of the important aspects of symmetric and p
key encryption. To
discriminate between the two, we will generally refer to the key used in symmetric
encryption as a
secret key
. The two keys used for public
key encryption are referred to the


public key
private key.
Invariably, the private key
is kept secret, but it is referred to as
a private key than a secret key to avoid confusion with symmetric encryption.

Let us take a closer look at the essential elements of a public
key encryption s
cheme, using
figure. There is some source A that produces a message in plaintext, X =
X[X1,X2,…..Xm]. The M elements of X are letters in some finite alphabet.

The message is intended for destination B. B generates a related pair of keys: a
public key, K
, and a private key, KR
. KR

is known only to B, whereas Ku

is publicly
available and therefore accessible by A.

With the message X and the encryption key KU

as input, A forms the ciphertext Y = Y
[Y1, Y2…YN]:

Y = E


Conventional Encryption Public
key Encryption

Needed to work :

Needed to Work :

1) The same algorithm with the same key 1) One algorithm is used for encryption

is used for encryption and decryption

and decrypt
ion with a pair of keys,

one for encryption and one for



The sender an
d receiver must share 2) The sender and receiver must each

the algorithm and the key. Have one of the matched pair of


the same one ).

Needed for Security :

Needed for Security :

1) The key must be kept secret. 1) One of the two keys must be kept



It may be impossible or at least 2) It may be impossible or at least

impractical to decipher a message if impractical to decipher a message

no other information is available.

If no other information is available.


Knowledge of the algorithm plus 3) Knowledge of the algorithm plus of

samples of ciphertext must be the keys plus samples of ciphertext

t to determine the key. must be insufficient to determine the

other key.



The intended receiver, in possessing of the matching private key, is able to invert the

X = D


key cryptosystem: secrecy

An opponent, observing Y and having access to KU
, but not having access to KR

or X,
must attempt to recover X and/or KR
. It is assumed that the opponent does have
knowledge of the encryption (E) and decryption (D) algorith
ms. If the opponent is
interested only in this particular message, then the focus of effort is to recover X, by
generating a plaintext estimate X^. Often, however, the opponent is interested in being able
to read future messages as well, in which case an a
ttempt is made to recover KR

generating an estimate K^R

We mentioned earlier that either of the two related keys can be used for encryption,
with the other being used for decryption. This enables a rather different cryptographic
scheme to be implem
ented. Whereas the scheme illustrated in Figure provides
confidentiality, Figure shows the use of public
key encryption to provide authentication:

Y = E


X = D


In this case, A prepares a message to B and encrypts it using A's privat
e key before
transmitting it. B can decrypt the message using A's public key. Because the message was
encrypted using A's private key, only A could have prepared the message. Therefore, the
entire encrypted message serves as a digital signature. In addit
ion, it is impossible to alter



Source A

Source A





Key pair


Destination B


B’s public key

B’s private



the message without access to A's private key, so the message is authenticated both in
terms of source and in terms of data integrity.

key Cryptosystem: Authentication

In the preceding scheme,
the entire message is encrypted, which, although validating both
author and contents, requires a great deal of storage. Each document must be kept in
plaintext to be used for practical purposes. A copy also must be stored in ciphertext so that
the origin a
nd contents can be verified in case of a dispute. A more efficient way of
achieving the same results is to encrypt a small block of bits that is function of the
document. Such a block, called an authenticator, must have the property that it is infeasible
o change the document without changing the authenticator. If the authenticator is
encrypted with the sender's private key, it serves as a signature that verifies origin, content,
and sequencing.

It is important to emphasize that the encryption proce
ss just described does not provide
confidentiality. That is, the message being sent is safe from alteration but not from
eavesdropping. This is obvious in the case of a signature based on a portion of the message,
because the rest of the message is transmi
tted in the clear. Even in the case of complete
encryption, as shown in figure, there is no protection of confidentiality because any
observer can decrypt the message by using the sender's public key.

It is, however, possible to provide both the authentic
ation function and
confidentiality by a double use of the public
key scheme.

Z = E

[ E
(X) ]

X = D
[ D
(z) ]



Source A

Source A





Key pair



Destination B

Destination B

A’s private


A’s public




Public _ key cryptosystem: Secrecy and Authentication

In this case, we being as before
by encrypting a message, using the sender's private
key. This provides the digital signature. Next, we encrypt again, using the receiver's public
key. Only the intended receiver, who alone has the matching private key, can decrypt the
final ciphertext. Thu
s, confidentiality is provided. The disadvantage of this approach is that
the public
key algorithm, which is complex, must be exercised four times rather than two
in each communication.

Application for Public
Key Cryptosystems:

Before proceeding, we n
eed to clarify one aspect of public
key cryptosystems that is
otherwise likely to lead to confusion, Public
key systems are characterized by the use of a
cryptographic type of algorithm with two keys, one held private and one available publicly.
on the application, the sender uses either the sender's private key or the
receiver's public key, or both, to perform some type of cryptosystems into three categories.











Key Pair


Key Pair


B’s priva


B’s public key
A’s public key

Source A

Destination B

A’s private





The sender encrypts a message with the recipient's public key

Digital signature:

The sender " signs " a message with its private key. Signature is achieved by a
cryptographic algorithm applied to the message of to a small block of data that is a function
of the message.

Key exchange:

Two sides cooperate to exch
ange a session key. Several different approaches are possible,
involving the private key(s)of one both parties.

Some algorithms are suitable for all three applications, whereas others can be used only for
one or two of these applications.


The pioneering paper by Diffie and Hellman [DIFF 76 b] introduce a new

Approach to cryptography and, in effect challenged cryptologists to come up with a
cryptographic algorithm that met the requirements for public

key systems. On
e of the first
of the responses to the challenge was developed in 1977 by Ron Rivest, Adi Shamir, and
Len Adleman at MIT and first published in 1978 [RIVE 78] the Rivest


(RSA) scheme has since that time reigned supreme as the most widely
accepted and
implemented general

purpose approach to public

key encryption.

The RSA scheme is a block cipher in which the plaintext and ciphertext are
integers between 0 and n
1 for some n. A typical size for n is 1024 bits, or 309 decimal
We examine RSA in this section in some detail, beginning with an explanation of
the algorithm. Then we examine some of the computational and cryptanalytical
implications of RSA.

Description of the Algorithm:

The scheme developed by Rives
t, Shamir, and Adleman makes use of an expression
with exponential. Plaintext is encrypted in blocks, with each block having a binary value
less than some number n. That is the block size must be less than or equal to log2(n); in
practice, the block size

is k bits, where 2k < n < 2k+1. Encryption and decryption are of
the following forms, for some plaintext block M and ciphertext block C.

C = Me mod n

M = Cd mod n = (Me) d mod n = Med mod n



Both sender and receiver must know the value of n. The
sender knows the value of e, and
only the receiver knows the value of d. Thus, this is a public
key encryption algorithm with
a public key of KU = {e,n} and a private key of KR ={d,n}. For the algorithm to be
satisfactory for public
key encryption, the fol
lowing requirements must be meet:

> it is possible to find value of e, d, n such that Med = M mod n for all M < n.

> it relatively easy to calculate Me and Cd for all values of M < n.

> it is infeasible to determine d given e and n.

For now, w
e focus on the first requirement and consider the other questions later. We need
to find a relationship of the form

Med = M mod n

A corollary to Euler's theorem, fits the bill: Given two prime numbers, p and q and two
integers n and m, such that n = p
q and 0 < m< n, and arbitrary integer k, the following
relationship holds:


(n) + 1 = mk (p
1)+1 = m mod n


(n) is the Euler totient function which is the number of positive integers less then n
and relatively prime to n. for p, q prime,

pq) = (p
1). Thus we can achieve the
desired relationship if

Ed = k

(n) + 1

This is equivalent to saying:

Ed = 1 mod


D = e
1 mod


That is e and d are multiplicative inverses mod

(n). Note that according to the rules of
modular arit
hmetic, this is true only if d (and therefore e) is relatively prime to

Equivalently, gcd (

(n), d) = 1

We are now ready to state the RSA scheme. The ingredients are the following:

P, q, two prime numbers

(private, chosen)

n = pq

(public, calculated)

e, with gcd(

(n),e) = 1; 1<e<


(public, chosen)

d = e
1 mod


(private, calculated)

The private key consists of {d, n} and the public key consists of {e, n}. Suppose that user
A has published its public key and tha
t user B wishes to send the message M to A. then B
calculates C = Me (mod m) and transmits C. on receipt of this ciphertext, user A decrypts
by calculating M = Cd (mod m).

It is worthwhile to summarize the justification for this algorithm. We have chose
n e and d
such that

d = e
1 mod



ed = 1 mod


Therefore, ed is of the form k

(n)+1. But by the corollary to Euler’s theorem, provided
here, given two prime numbers p and q, and integers n = pq and M with

0 < M < m:


(n) + 1 = Mk (p
1)+1 = M mod n



So, Med = M mod n.


C = Me mod n

M = Cd mod n = (Me) d mod n = Med mod n = m mod n

Advantages & Benefits:

ClassicSys as a standard...

Besides ClassicSys ciphering at high speed, two more advantage
s make

Classic prime candidate for THE standard application in cryptography :


ClassicSys uses only 1 secret key to meet ALL the cryptographic needs of an

user such as :

To authenticate himself

To authenticate messages with a time reference

To ge
nerate all the Session Keys he needs for Email (as one possible

To generate several keys for other applications: banking, electronic
commerce, electronic voting, casino games at home, ...

2. ClassicSys is designed in such a way that there i
s no valid reason to forbid it's

use in any country in the world. ClassicSys gives all the required guarantees to its

users and their government : secret keys must not be divulged and Security Services

can always decipher suspect messages.


& benefits for the End
User ...

ClassicSys offers more than the known advantages of encryption solutions:

Very high speed of encryption (see below).

The chip contains the SED algorithm and all the other features of
ClassicSys. One system covers all cr
yptographic needs, for all applications.

New applications can be added without updating the chip.

ClassicSys works is fully automated, requests to the TA are returned
directly, without human intervention.

Private Keys are completely unknown to everybody,

even the Trust
Authority's manager! All keys are written into chips and are not accessible to humans or
other machines. This guarantees the privacy of all the end

Once an end
user has received the information to generate his Application
Keys, he do
es not need the intervention of the TA anymore. Email for example, users do
not need the TA to exchange messages between themselves.

ClassicSys acts like a public key cryptosystem : every end
user has one
public ID number, which is used in a similar way to

public keys. Email for example, when


somebody wants to communicate with another end
user, he sends to the TA his ID number
and the one from his correspondent. In return he receives information from the TA to
generate their Session Key.

Advantages & ben
efits for the Authority...

ClassicSys enables the TA and National Security Service (NSS) to act
completely separately, under different authorities, as required by our Democracies.
Requests from the NSS to the TA are recorded encrypted by the TA (TA doesn'
t know the
ID of Alice or Bob in a suspect message). This guarantees the confidentiality of the NSS's
investigation, however, the recorded provides an audit trail for any Competent
Investigating Authority. Optimum ClassicSys operation should have the

TA and NSS
under different authorities, but every country can implement it as seen fit.

ClassicSys enables the NSS to decrypt the content of suspect incoming and
outgoing international messages, without the necessity for users to deposit their private
cret keys in the corresponding countries (as with the RSA).

Only the NSS is able to request necessary information to the TA to
investigate suspect messages.

Each country remains independent regarding the deciphering of the
incoming and outgoing messages:

each message contains the necessary information to be
deciphered by the 2 National Security Services.

Each Trust Authority has its own Private Key. Consequently they can only
compute Private Keys for domestic users.

Technical advantages & benefits

ssicSys is easy to implement in integrated circuits because:

It uses only XOR and branching functions

No reporting arithmetic bits are needed

Programming can be done with a polynomial structure.

The length of the blocks of key and data are identical an
d equal to 128 bits
(16 bytes).

Security of ClassicSys is enhanced compared to other systems because:

Deciphering is not the reverse of ciphering

The ciphering and deciphering keys are different

All the PrivateKeys (end
users, TAs, NSS’s) are included

in an IC and
therefore not accessible.

There is no known way to reconstruct, by cryptanalysis, the secret key, knowing a

clear and it's corresponding encrypted message.

Differential cryptanalysis is not suitable to the SED algorithm. On average, there


only one key corresponding to a clear and its associated encrypted text and therefore, each
bit of the key has equal weight in the algorithm.



Only 1 secret key of 128 bits is enough to meet all the cryptographic needs of an

user such as :

generate all the Session Keys he needs

To authenticate himself

To authenticate messages with a time reference

To generate several keys for other applications (banking, electronic

commerce, electronic voting, casino games at home,...)

ke the RSA algorithm, where every key requires a determined space, the SED

algorithm can use every block contained in the space 2128.

The SED algorithm is very fast for the following reasons:

The length of the blocks (key and data) is small (128 bits ag
ainst more than
512 bits) but long enough to disable every exhaustive cryptanalysis.

On average. It is possible to compute at 1/3 of the clock frequency (8 to 10

The SED algorithm is completely transparent. Due to the theory of Multiplicati

Groups we can confirm that there is no Trojan Horse in the SED algorithm.

The SED algorithm permits chained mode ciphering, allowing reduction of the

authentication information to one block of 128 bits, whatever the length of the data

to authentica

Comparison between the DES, the RSA and the SED

The table below compares the important features of the DES, the RSA and the SED
algorithms, used within global cryptographic systems.

Feature DES



high low high

Deposit of keys

needed needed not needed

Country independ

no no yes

Trojan Horse

not proved no no

Data block length

64 bits minimum 512 bits 128 bit

Key length

56 bits minimum 512 bits 128 bits

Use of data space

full, 64 bits (2^64), variable, limited, full 128 bits

8 bytes not defined (2^128), 16


Ciphering & deciphering


same different different

Ciphering & deciphering


different same different

Algorithm contains only

R and branching

no no yes



Average number of key

For one pair E&C=1

probably not probably yes yes

cryptanalysis method

differential method produc
t no known

factorization method

Global system including


not suitable not suitable ClassicSys


Cryptography is extremely useful; there is a multitude of applications, many of which are

in use. A typical application of cryptography is a system built out of the basic
techniques. Such systems can be of various levels of complexity. Some of the more simple
applications are secure communication, identification, authentication, and secret sha
More complicated applications include systems for electronic commerce, certification,
secure electronic mail, key recovery, and secure computer access.

In general, the less complex the application, the more quickly it becomes a reality.
ion and authentication schemes exist widely, while electronic commerce systems
are just beginning to be established. However, there are exceptions to this rule; namely, the
adoption rate may depend on the level of demand. For example, SSL
encapsulated HTTP

(see Question 5.1.2) gained a lot more usage much more quickly than simpler link
encryption has ever achieved. The adoption rate may depend on the level of demand.

Secure Communication

Secure communication is the most straightforward use of crypt
ography. Two people may
communicate securely by encrypting the messages sent between them. This can be done in
such a way that a third party eavesdropping may never be able to decipher the messages.
While secure communication has existed for centuries, the

key management problem has
prevented it from becoming commonplace. Thanks to the development of public
cryptography, the tools exist to create a large
scale network of people who can
communicate securely with one another even if they had never communi
cated before.

Identification and Authentication

Identification and authentication are two widely used applications of cryptography.
Identification is the process of verifying someone's or something's identity. For example,
when withdrawing money from a
bank, a teller asks to see identification (for example, a
driver's license) to verify the identity of the owner of the account. This same process can be
done electronically using cryptography. Every automatic teller machine (ATM) card is
associated with a
``secret'' personal identification number (PIN), which binds the owner to
the card and thus to the account. When the card is inserted into the ATM, the machine
prompts the cardholder for the PIN. If the correct PIN is entered, the machine identifies that
erson as the rightful owner and grants access. Another important application of


cryptography is authentication. Authentication is similar to identification, in that both allow
an entity access to resources (such as an Internet account), but authentication
is broader
because it does not necessarily involve identifying a person or entity. Authentication
merely determines whether that person or entity is authorized for whatever is in question.
For more information on authentication and identification, see Ques
tion 2.2.5.

Secret Sharing

Another application of cryptography, called secret sharing, allows the trust of a secret to be
distributed among a group of people. For example, in a (k, n)
threshold scheme,
information about a secret is distributed in such a

way that any k out of the n people (k £ n)
have enough information to determine the secret, but any set of k
1 people do not. In any
secret sharing scheme, there are designated sets of people whose cumulative information
suffices to determine the secret.
In some implementations of secret sharing schemes, each
participant receives the secret after it has been generate.


This document's some topics are just picked up by some of reference book and
some excellent web sight which give me good

explore such references are following.

Cryptography And Network Security (William Stallings).

Computer Network ( Andrew S. Tanenbaum).

Conclusion :

By analysis of this report and their su
btopics which are mentioned above, which are
inherently guides us about various cryptographic techniques used in data security. By using
of encryption techniques a fair unit of confidentiality, authentication, integrity, access
control and availability of
data is maintained. Using cryptography Electronic Mail Security,
Mail Security, IP Security, Web security can be achieved.