DIPLOMA IN CYBER SECURITY

DCS-03
Information Security

Block

1 INFORMATION SECURITY CONCEPTS

AND CRYPTOGRAPHY

Unit – 1
Information Security Concepts

Unit – 2
Introduction to Cryptography

Unit – 3
Cryptographic Algorithms
 EXPERT COMMITTEE
Dr P.K.Behera (Chairman)
Reader in Computer Science
Utkal University
Bhubaneswar, Odisha
Dr. J.R.Mohanty (Member)
Professor and HOD
KIIT University
Bhubaneswar, Odisha
Sh Pabitrananda Pattnaik(Member)
Scientist –E,NIC
Bhubaneswar, Odisha
Sh Malaya Kumar Das (Member)
Scientist –E,NIC
Bhubaneswar, Odisha
Dr. Bhagirathi Nayak (Member)
Professor and Head(IT & System)
Sri Sri University
Bhubaneswar, Odisha
Dr.Manoranjan Pradhan (Member)
Professor and Head(IT & System)
G.I.T.A
Bhubaneswar, Odisha
Sri V.S.Sandilya (Convenor)
Academic Consultant (I.T)
Odisha State Open University,
Sambalpur, Odisha

DIPLOMA IN CYBER SECURITY

Course Writer
Chandrakant Mallick
Consultant (Academic)
School of Computer and Information Science
Odisha State Open University, Odisha

1st Edition - 2016 Printed at - Shree Mandir Publications, BBSR

UNIT-1
INFORMATION SECURITY CONCEPTS Unit Structure

1.0 Introduction
1.1 Learning Objectives
1.2 Information Security issues
1.3 Basic Terminologies and Definitions
1.4 Information Security Goals
1.5 Challenges of Information Security
1.6 The OSI Security Architecture
1.7 Types of Security Attacks
1.7.1 Attacks on Confidentiality
1.7.2 Attacks on Integrity
1.7.3 Attacks on Availability
1.8 Classifications of Security attacks
1.8.1 Passive Attacks
1.8.2 Active Attacks
1.8.2.1 Types of active Attacks
1.8.3 Passive versus Active Attacks
1.9 Self-Assessment Questions-1
1.10 Security Services and mechanisms
1.10.1 Security Services
1.10.2 Security Mechanisms
1.10.3 Specific Security Mechanisms
1.10.4 Pervasive Security Mechanisms
1.11 Relation between security services and mechanisms
1.12 Self-Assessment Questions-2
1.13 Key terms & Concepts
1.14 Answer to Self-Assessment Questions-1
1.15 Answer to Self-Assessment Questions-2
1.16 References and Suggested Readings

Odisha State Open University Page 1

 1.0 INTRODUCTION

Computers are used by millions of people for many purposes like Banking,
Shopping, Tax returns, Education, Military, Communication, Business, Research,
Entertainment and many more. It necessitates protecting the assets of computer in
all such applications. The assets of the computer mean the hardware and software
including the information available in many forms. In the present day context
information is the most valuable asset than any other in an organization. Information
Security deals with all aspects of protecting information. In this unit we will discuss
some fundamentals concepts of information security.

1.1 LEARNING OBJECTIVES

After studying this unit, you should be able to:

· Understand the concepts of security and its types.
· Understand what information security is and how it plays an important role
today.
· Distinguish the meaning of computer security, information security and
network security.
· Describe the key security requirements of confidentiality, integrity, and
availability.
· Discuss the types of security threats and attacks.
· Understand the X.800 security architecture for OSI
· Summarize the key terms and critical concepts of information security.

1.2 INFORMATION SECURITY ISSUES

The information security within an organization has undergone two major changes
in the last several decades.
The world before computers was much simpler in some ways-
· Signing, legalizing a paper would authenticate it.
· Photocopying was easily detected
· Erasing, inserting, modifying words on a paper document easily detectable

Page 2 Odisha State Open University

· Secure transmission of a document: seal it and use a reasonable mail carrier
(hoping the mail train does not get robbed)
· One can recognize each other's face, voice, hand signature, etc.
In the world of computers the ability to copy and alter information has changed
dramatically as-
· There is no difference between an “original” file and copies of it.
· Removing a word from a file or inserting others is undetectable
· Adding a signature to the end of a file/email: one can impersonate it –add it
to other files as well, modify it, etc.
· Difficult to authenticate the person electronically communicating with you.
So, it needs automated tools for protecting files and other information stored on the
computer.

1.3 BASIC TERMINOLOGIES AND DEFINITIONS

Cyber security or information technology security focuses on protecting

computers, networks, programs and data from unintended or unauthorized access,
change or destruction.
Governments, military, corporations, financial institutions, hospitals and other
businesses collect, process and store a great deal of confidential information on
computers and transmit that data across networks to other computers. With the
growing volume and sophistication of cyber attacks, ongoing attention is required
to protect sensitive business and personal information, as well as safeguard
national security.
So Information Security is a crucial issue in many applications. The basic
terminologies similar to information security are discussed as follows.
Computer security is a collection of tools designed to protect the assets of the
computer system; information and services they provide.

Computer Security (Definition)

The NIST Computer Security Handbook [NIST95] defines the term computer
security as follows:
The protection afforded to an automated information system in order to attain the
applicable objectives of preserving the integrity, availability and confidentiality of

Odisha State Open University Page 3

 information system resources (includes hardware, software, firmware,
information/data, and telecommunications) is called Computer Security.
The objective of computer security includes protection of information and property
from theft, corruption, or natural disaster, while allowing the information and
property to remain accessible and productive to its intended users.
Another major change that affected security is introduction of computer networks
and distributed systems that facilitates carrying of data from one computer to the
other. So it is even more essential to provide security to information for the systems
that access information over public telephone networks or data networks or the
Internet. The measure that is needed to protect data during their transmission is
called Network Security.
Network security mechanisms need to protect the network and the network-
accessible resources from unauthorized access by consistent and continuous
monitoring and measurement of its effectiveness.
Now a day's virtually all business, government and academic institutions
interconnect their data processing systems with collection of interconnected
networks called Internet. The mechanism used to determine, prevent, detect and
correct security violations that involve transmission of information is called
Internet Security.
Similarly the mechanism to make sure that nosy people cannot read or secretly
modify or disclose the information is called Information Security.
Information security, sometimes shortened to InfoSec, is the practice of defending
information from unauthorized access, use, disclosure, disruption, modification,
inspection, recording or destruction. It is a general term that can be used regardless
of the form the data may take (e.g. electronic, physical).
Information security (Definition)
Wikipedia defines Information security (sometimes shortened to InfoSec), as the
practice of defending information from unauthorized access, use, disclosure,
disruption, modification, perusal, inspection, recording or destruction.
It is a general term that can be used regardless of the form the data may take (e.g.
electronic, physical).
Point-to-Make: Making a network or a communication system secure involves
more than just keeping it free of programming errors as it involves often intelligent,

Page 4 Odisha State Open University

dedicated and well-funded adversaries.
Possible adversaries
An adversary is generally considered to be a person, group, or force that opposes
and/or attacks.
· A Student is interested to have fun snooping on other people's email
· A Cracker tries to test out someone's security system, to steal data
· A Businessman wants to discover a competitor's strategic marketing plan
· An Ex-employee wants to get revenge for being fired.
· An Accountant interested to embezzle money from a company
· A Stockbroker wants to deny a promise made to a customer by email
· A Convict to steal credit card numbers for fraudulent actions.
· A Spy wants to learn an enemy's military or industrial secrets
· A Terrorist wants to use internet for attacks

1.4 INFORMATION SECURITY GOALS

The meaning of terminologies change over time and this is especially true in the
rapidly changing technology industry. For example, we have information security,
computer security, cyber security and IT security.
Not only have these names changed meaning over time, there isn't necessarily a
clear consensus on the meanings and the degree to which they overlap or are
interchangeable. In our context we will frequently refer information security in this
course.
As per the definition above there are three measure goals of Information security
that include Confidentiality, Integrity and availability.

Fig:Information Security goals

Odisha State Open University Page 5

 1. Confidentiality
It covers two related concepts to ensure:
Data confidentiality: Assures that private or confidential information is not made
available or disclosed to unauthorized individuals.
Privacy: Assures that individuals control or influence what information related to
them may be collected and stored and by whom and to whom that information may
be disclosed.
Confidentiality or Secrecy requires that the information in a computer system only
be accessible for reading by authorized parties.
This type of access includes:
· Printing
· Displaying
· Other forms of disclosure, including simply revealing the existing of
an object
We need to protect our information from malicious actions that threat confidentiality
of its information.
The principle of confidentiality specifies that only the sender and the intended
receiver should be able to access the message.
Confidentiality gets compromised if an unauthorized person is able to access the
message.
2. Integrity
Information has integrity when it is timely, accurate, complete, and consistent.
However, computers are unable to provide or protect all of these qualities. The
term Integrity covers two related concepts:
(i) Data integrity is a requirement that information and programs are
changed only in a specified and authorized manner.
(ii) System integrity is a requirement that a system “performs its intended
function in an unimpaired manner, free from deliberate or inadvertent
unauthorized manipulation of the system.”
The definition of integrity has been, and continues to be, the subject of much debate
among computer security experts.
In context of information security, we mean information has integrity it has not been
altered i.e. if the content of the message does not change during transmission the the
integrity are maintained.
If the content of the message changes during transmission then the integrity is lost.

Page 6 Odisha State Open University

The changes need to be done to the information only by the authorized user through
authorized mechanisms.
· Integrity requires that the computer system asset can be modified only by
authorized parties.
· Modification includes:
· Writing
· Changing
· Changing status
· Deleting and
· Creating

3. Availability
Availability requires that computer system assets are available to authorized
parties.
Availability is arequirement intended to assure that systems work promptly and
service is not denied to anauthorized user. The principle of availability states the
resources should be available to authorized users all the times.These three
concepts form what is often referred to as the Confidentiality Integrity and
Availability (CIA) Triad.
The three concepts embody the fundamental security objectives for both data and
for information as well as computing services. For example, the NIST standard
FIPS 199 (Standards for Security Categorization of Federal Information and
Information Systems) lists confidentiality, integrity, and availability as the three
security objectives for information and for information systems.
FIPS 199 provides a useful characterization of these three objectives in terms of
requirements and the definition of a loss of security in each category:
Confidentiality: Preserving authorized restrictions on information access and
disclosure, including means for protecting personal privacy and proprietary
information.
A loss of confidentiality is the unauthorized disclosure of information.
Integrity: Guarding against improper information modification or destruction,
including ensuring information nonrepudiation and authenticity.
A loss of integrity is the unauthorized modification or destruction of information.
Availability: Ensuring timely and reliable access to and use of information.

Odisha State Open University Page 7

 A loss of availability is the disruption of access to or use of information or
aninformation system.
Other Security principles
Although the use of the CIA triad to define security objectives is well established,
some in the security field feel that additional concepts are needed to present a
complete picture. Three of the most commonly mentioned are as follows:
Authenticity: The property of being genuine and being able to be verified and
trusted; confidence in the validity of a transmission, a message, or message
originator. This means verifying that users are who they say they are and thateach
input arriving at the system came from a trusted source.
Authentication mechanisms help to establish proof of identities. Authentication
process ensures that the origin of an electronic message or a document is correctly
identified.
Accountability: The security goal that generates the requirement for actions of an
entity to be traced uniquely to that entity. This supports nonrepudiation, deterrence,
fault isolation, intrusion detection and prevention, and after action, recovery and
legal action. Because truly secure systems are not yet an achievable goal, we must be
able to trace a security breach to a responsible party. Systems must keep records of
their activities to permit later forensic analysis to trace security breaches or to aid in
transaction disputes.
Non-Repudiation: The user sends a message to B and can't deny that she had sent
that message. For example A could send a fund transfer request to the bank B over the
Internet. After the bank performs the fund transfer as per A's instruction, A could
claim that she never sent the fund transfer request. Thus A repudiates or denies her
fund transfer instruction.

1.5 CHALLENGES OF INFORMATION SECURITY

Information security is both complex and challenging. Some of the reasons are as
follows:
1. Security is not as simple as it might be understood by the novice users. The
requirements seem to be straight forward; the major requirements for security
services can be confidentiality, authentication, nonrepudiation, or integrity. But the
mechanisms used to meet those requirements can be quite complex.
2. In developing a particular security mechanism or algorithm, one must

Page 8 Odisha State Open University

alwaysconsider potential attacks on those security features. In many cases,
successful attacks are designed by looking at the problem in a completely different
way, therefore exploiting an unexpected weakness in the mechanism.
3. Typically, a security mechanism is complex, and it is not obvious from the
statement of a particular requirement that such elaborate measures are needed. It is
only when the various aspects of the threat are considered that elaborate security
mechanisms make sense.
4. Having designed various security mechanisms, it is necessary to decide where to
use them. This is true both in terms of physical placement and in a logical sense
(e.g., at what layer or layers of architecture such as TCP/IP should mechanisms be
placed).
5. Security mechanisms typically involve more than a particular algorithm
orprotocol. They also require that participants be in possession of some secret
information (e.g., an encryption key), which raises questions about the creation,
distribution, and protection of that secret information. There also may be a reliance
on communications protocols whose behavior may complicate the task of
developing the security mechanism. For example, if the proper functioning of the
security mechanism requires setting time limits on the transittime of a message
from sender to receiver, then any protocol or networkthat introduces variable,
unpredictable delays may render such time limits meaningless.
6. The attacker has is that he or she need only find a single weakness, while the
designer must find and eliminate all weaknesses to achieve perfect security.
7. There is a natural tendency on the part of users and system managers to perceive
little benefit from security investment until a security failure occurs.
8. Security requires regular, even constant, monitoring, and this is difficult in
today's short-term, overloaded environment.
9. Security is still too often an afterthought to be incorporated into a system after the
design is complete rather than being an integral part of the design process.
10. Many users and even security administrators view strong security as an
impediment to efficient and user-friendly operation of an information system or use
of information.

1.6 THE OSI SECURITY ARCHITECTURE

ITU-T Recommendation X.800, Security Architecture for OSI, defines a

systematic approach of defining the requirements for security and characterizing

Odisha State Open University Page 9

 the approaches to satisfying those requirements. The OSI security architecture is
useful to managers as a way of organizing the task of providing security.
Furthermore, because this architecture was developed as an international standard,
computer and communications vendors have developed security features for their
products and services that relate to this structured definition of services and
mechanisms.
The OSI security architecture focuses on security attacks, mechanisms, and services.
These can be defined briefly as follows.
Security attack: Any action that compromises the security of information owned by
an organization.
Security mechanism: A process (or a device incorporating such a process) that is
designed to detect, prevent, or recover from a security attack.
Security service: A processing or communication service that enhances the security
of the data processing systems and the information transfers of an organization. The
services are intended to counter security attacks, and they make use of one or more
security mechanisms to provide the service.
In the literature, the terms threat and attack are commonly used to mean more or less
the same thing. In RFC 4949, Internet Security Glossary, threats and attacks are
defined as follows.
Threat: A threat is a potential for violation of security, which exists when there is a
circumstance, capability, action, or event that could breach security and cause harm.
That is, a threat is a possible danger that might exploit vulnerability.
Attack: An assault on system security that derives from an intelligent threat; that is,
an intelligent act that is a deliberate attempt (especially in the sense of a method or
technique) to evade security services and violate the security policy of a system.

1.7 TYPES OF SECURITY ATTACKS

There are several types of security attacks but we can classify them based on the
security goals they compromise. The following figure shows the categories of
different types of security attacks.

Page 10 Odisha State Open University

 Fig: Taxonomy of attacks with relation to security goals

1.7.1 Attacks on Confidentiality

Snooping refers to unauthorized access to or interception of data.
Traffic analysis refers to obtaining some other type of information by monitoring
online traffic.
1.7.2 Attacks on Integrity
Modification means that the attacker intercepts the message and changes it.
Masquerading or spoofing happens when the attacker impersonates somebody
else.
Replaying means the attacker obtains a copy of a message sent by a user and later
tries to replay it.
Repudiation means that sender of the message might later deny that she has sent
the message; the receiver of the message might later deny that he has received the
message.
1.7.3 Attacks on Availability
Denial of Service (DoS) is a very common attack. It may slow down or totally
interrupt the service of a system.

1.8 CLASSIFICATIONS OF SECURITY ATTACKS

According to both X.800 and RFC 4949, security attacks are classified as passive
attacks and active attacks.

Odisha State Open University Page 11

 Attacks

Active Attacks Passive Attacks

Fig: Categories of attacks

1.8.1 Passive Attacks
A passive attack attempts to learn or make use of information from the system but
does not affect system resources. A passive attack is difficult to detect and isolate.
These do not involve any modification to the contents. So the approach should be to
prevent a passive attack rather than detection and corrective actions on it. Passive
attacks are of two types:

Passive Attacks

Release of message Traffic Analysis

contents

Fig: Classifications of Passive attacks

i) Release of message contents

The release of message contents means the message content may be disclosed
somehow during transit. A third party may passively capture the message content
without the knowledge of the sender and the receiver. A telephone conversation, an
electronic mail message, and a transferred file may contain sensitive or confidential
information. We would like to prevent an opponent from learning the contents of
these transmissions.
ii) Traffic Analysis refers to obtaining some other type of information by monitoring
online traffic. Suppose that wehad 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 for masking
contents is encryption. If we had encryption protection in place, an opponent might
still be able to observe the pattern of these messages. The opponent could determine
the location and identity of communicating hosts and could observe the frequency
and length of messagesbeing exchanged. This information might be useful in
guessing the nature of the communication that was taking place.
1.8.2 Active Attacks:
An active attack attempts to alter system resources or affect their operation. These

Page12 Odisha State Open University

attacks involve some modifications of original message in some manner or
creation of false messages.
It is hard to prevent the active attacks. It is rather easier to detect and recover from
them.
1.8.2.1 Types of active Attacks
Active attacks involve some modification of the data stream or the creation of a
false stream and can be subdivided into four categories:
Masquerade, replay, modification of messages, and denial of service.

Active Attacks

Masquerade Replay Modification Denial of

Services

Fig: Classifications of active attacks

A masquerade takes place when one entity pretends to be a different entity. A

masquerade attack usually includes one of theother forms of active attack. For
example, authentication sequences can be capturedand replayed after a valid
authentication sequence has taken place, thus enabling anauthorized entity with
few privileges to obtain extra privileges by impersonating anentity that has those
privileges.
Replay involves the passive capture of a data unit and its subsequent
retransmissionto produce an unauthorized effect.
Modification of messages simply means that some portion of a legitimatemessage
is altered, or that messages are delayed or reordered, to produce anunauthorized
effect.
The denial of service prevents or inhibits the normal use or management
ofcommunications facilities. This attack may have a specific target; forexample, an
entity may suppress all messages directed to a particular destination (e.g., the
securityaudit service). Another form of service denial is the disruption of an entire
network, either by disabling the network or by overloading it with messages so as to
degrade performance.
1.8.3 Passive versus Active Attacks
Active attacks present the opposite characteristics of passive attacks. Whereas
passive attacks are difficult to detect, measures are available to prevent their
success.On the other hand, it is quite difficult to prevent active attacks absolutely

Odisha State Open University Page 13

 because of the wide variety of potential physical, software, and network
vulnerabilities.Instead, the goal is to detect active attacks and to recover from
any disruption or delays caused by them. If the detection has a deterrent
effect, it may also contribute to prevention.
Different types of attacks that threaten the security goals are explained in the
following table.

Table: Passive versus Active Attacks

Attacks Active/Passive Threatening
Snooping Passive Confidentiality
Traffic Analysis
Masquerading, Replay, Active Integrity
Modification, Repudiation
Denial of Services Active Availability

1.9 SELF-ASSESSMENT QUESTIONS-

1Answer the following questions in brief.

1. What are the three goals of Information Security?
………………………………………………………………………………………
………………………………………………………………………………………
…………………………………………………………………………............……
2. Identify different categories of active attacks
………………………………………………………………………………………
………………………………………………………………………………………
……………………………………………………………………............…………
3. Identify different categories of passive attacks.
………………………………………………………………………………………
………………………………………………………………………………………
…………………............……………………………………………………………
4. What is the difference between passive and active attacks?
………………………………………………………………………………………
………………………………………………………………………………………
………………………………………………………………………............………
5. Name the security attacks on confidentiality.
………………………………………………………………………………………
………………………………………………………………………………………
……………..….........................................................................................................
Page 14 Odisha State Open University
6. Name the security attacks on integrity
……………………………………………………………………………………
……………………………………………………………………………………
………………….................................................................................................…
7. Name the security attacks on availability.
………………………………………………………………………...…………
……………………………………………………………………………………
…………………….................................................................................................

1.10 SECURITY SERVICES AND MECHANISMS

ITU-T provides some security services and some mechanisms to implement those
services. Security services and mechanisms are closely related because a
mechanism or combination of mechanisms is used to provide a service.
1.10.1 Security Services
Security ServicesX.800 divides these services into five categories and fourteen
specific services

Fig: Categories of Security Services

Odisha State Open University Page 15

 Authentication
The authentication service is concerned with assuring that a communication
isauthentic. In the case of a single message, such as a warning or alarm signal,
thefunction of the authentication service is to assure the recipient that the messageis
from the source that it claims to be from. In the case of an ongoing interaction, such as
the connection of a terminal to a host, two aspects are involved. First, at the time of
connection initiation, the service assures that the two entities are authentic, that is,
that each is the entity that it claims to be. Second, the servicemust assure that the
connection is not interfered with in such a way that a thirdparty can masquerade as
one of the two legitimate parties for the purposes ofunauthorized transmission or
reception.Two specific authentication services are defined in X.800.
Peer entity authentication: Provides for the corroboration of the identityof a peer
entity in an association. Two entities are considered peers if theyimplement to same
protocol in different systems; for example two TCP modulesin two communicating
systems. Peer entity authentication is provided foruse at the establishment of, or at
times during the data transfer phase of, aconnection. It attempts to provide
confidence that an entity is not performingeither a masquerade or an unauthorized
replay of a previous connection.
Data origin authentication: Provides for the corroboration of the source of adata
unit. It does not provide protection against the duplication or modificationof data
units. This type of service supports applications like electronic mail, where there are
no prior interactions between the communicating entities.
Access Control
In the context of network security, access control is the ability to limit and
controlthe access to host systems and applications via communications links. To
achievethis, each entity trying to gain access must first be identified, or
authenticated, sothat access rights can be tailored to the individual.
Data Confidentiality
Confidentiality is the protection of transmitted data from passive attacks. With
respect to the content of a data transmission, several levels of protection can be
identified. The broadest service protects all user data transmitted between twousers
over a period of time. For example, when a TCP connection is set up between two
systems, this broad protection prevents the release of any user data transmitted over
the TCP connection. Narrower forms of this service can also be defined, including the
protection of a single message or even specific fields within a message.
These refinements are less useful than the broad approach and may even be

Page 16 Odisha State Open University

morecomplex and expensive to implement.The other aspect of confidentiality is the
protection of traffic flow from analysis.This requires that an attacker not be able to
observe the source and destination, frequency,length, or other characteristics of the
traffic on a communications facility.
Data Integrity As with confidentiality, integrity can apply to a stream of messages,
a single message, or selected fields within a message. Again, the most useful and
straightforwardapproach is total stream protection.
A connection-oriented integrity service, one that deals with a stream of messages,
assures that messages are received as sent with no duplication, insertion,
modification, reordering, or replays. The destruction of data is also covered
underthis service. Thus, the connection-oriented integrity service addresses both
message stream modification and denial of service. On the other hand, a
connectionless integrity service, one that deals with individual messages without
regard to any largercontext, generally provides protection against message
modification only.
We can make a distinction between service with and without recovery.Because the
integrity service relates to active attacks, we are concerned with detection rather
than prevention. If a violation of integrity is detected, then the servicemay simply
report this violation, and some other portion of software or human intervention is
required to recover from the violation. Alternatively, there are mechanisms
available to recover from the loss of integrity of data, as we will
reviewsubsequently. The incorporation of automated recovery mechanisms is, in
general, the more attractive alternative.
Non-repudiation
Nonrepudiation prevents either sender or receiver from denying a transmitted
message.Thus, when a message is sent; the receiver can prove that the alleged
sender infact sent the message. Similarly, when a message is received, the sender
can provethat the alleged receiver in fact received the message.
Availability Services
Both X.800 and RFC 4949 define availability to be the property of a system or
asystem resource being accessible and usable upon demand by an authorized
systementity, according to performance specifications for the system (i.e., a system
is available if it provides services according to the system design whenever users
requestthem). A variety of attacks can result in the loss of or reduction in
availability. Someof these attacks are amenable to automated countermeasures,
such as authenticationand encryption, whereas others require some sort of physical

Odisha State Open University Page 17

 action to preventor recover from loss of availability of elements of a distributed
system.
1.10.2 Security Mechanisms
As defined in X.800, thesecurity mechanisms aredivided into those that are
implemented in a specific protocol layer, such as TCP or an application-layer
protocol, and those that are not specific to any particular protocol layer or security
service. X.800 distinguishes between reversible encipherment mechanisms and
irreversible encipherment mechanisms. A reversible encipherment mechanism is
simply an encryption algorithm that allows data to be encrypted and subsequently
decrypted. Irreversible encipherment mechanisms include hash algorithms and
message authentication codes, which are used in digital signature and message
authentication applications.
1.10.3 Specific Security Mechanisms
Some specific security mechanism may be incorporated into the appropriate
protocollayer in order to provide some of the OSI securityservices.
Encipherment
The use of mathematical algorithms to transform data into a form that is not readily
intelligible. The transformation and subsequent recovery of the data depend on an
algorithm and zero or moreencryption keys.
Digital Signature
Data appended to, or a cryptographic transformation of, a data unit that allows a
recipient of the data unitto prove the source and integrity of the data unit and protect
against forgery (e.g., by the recipient).
Access Control
A variety of mechanisms that enforce access rights to resources.
Data Integrity
A variety of mechanisms used to assure the integrity of a data unit or stream of data
units.
Authentication Exchange
A mechanism intended to ensure the identity of an entity by means of information
exchange.
Traffic Padding
The insertion of bits into gaps in a data stream to frustrate traffic analysis attempts.
Routing Control
Enables selection of particular physically secure routes for certain data and allows
routing changes, especially when a breach of security is suspected.

Page 18 Odisha State Open University

Notarization
The use of a trusted third party is to assure certain properties of a data exchange.

Fig: Security Mechanisms

1.10.4 Pervasive Security Mechanisms

Mechanisms those are not specific to any particularOSI security service or protocol
layer.
Trusted Functionality
That which is perceived to be correct with respectto some criteria (e.g., as
established by a security policy).
Security Label
The marking bound to a resource (which may be a data unit) that names or
designates the security attributesof that resource.
Event Detection

Odisha State Open University Page 19

 It means the detection of security-relevant events.
Security Audit Trail
Data collected and potentially used to facilitate asecurity audit, which is an
independent review andexamination of system records and activities.
Security Recovery
Deals with requests from mechanisms, such as event handling and management
functions, and takes recovery actions.

1.11 RELATION BETWEEN SECURITY SERVICES AND MECHANISMS

The relationship between security services and mechanisms are summarized in the
table below:

Table: R elationship between Security Services and Mechanisms

Security Services Security Mechanisms
Data confidentiality E ncipherment and routing control
Data Integrity E ncipherment, Digital Signature, Data integrity
Authentication E ncipherment, Digital Signature, Authentication
E xchanges
Nonrepudiation Digital Signature, Data integrity and notarization
Access Control Access control m echanism

1.12 SELF-ASSESSMENT QUESTIONS-2

1. What are the key challenges of Information Security?
……………………………………………………………………………………
……………………………………………………………………………………
…………...................................................................................................…………

2. What are Passive Attacks? Why are they difficult to detect? Name some
passive attacks.
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………........………………………………………………………………….
3. List and briefly define categories of security services.
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………

Page 20 Odisha State Open University

1.13 KEY TERMS AND CONCEPTS

· Computer Security is concerned with protecting the assets of the computer

system; information and services they provide.
· Network Security is the measures to protect data during their transmission.
· Internet Security is the measures to protect data during their transmission
over a collection of interconnected networks called Internet.
· Information Security is about to prevent attacks or failing that to detect
attacks on information based schemes/systems.
· Security attack means any action that compromises the security of
information owned by an organization.
· Security mechanism is a mechanism that is designed to detect, prevent or
recover from a security attack.
· Security service means a service that enhances the security of the data
processing systems and the information transfers of an organization. The services
are intended to counter security attacks and they make use of one or more security
mechanisms to provide the service.

1.14 ANSWER TO SELF-ASSESSMENT QUESTIONS-1

1. The three security goals are confidentiality, integrity, and availability.
Confidentiality means protecting confidential information.
Integrity means that changes to the information need to be done only by
authorized entities.
Availability means that information needs to be available to authorized entities.
2. Active attacks involve some modification of the data stream or the creation of a
false stream and can be subdivided into four categories: masquerade, replay,
modification of messages, and denial of service.
3. Passive attack attempts to learn or make use of information from the system but
does not affect system resources. A passive attack is difficult to detect and
isolate. Passive attacks are of two types: Release of message contents and
Traffic Analysis.
4. Passive attacks are difficult to detect, measures are available to prevent their
success. On the other hand, it is quite difficult to prevent active attacks
absolutely because of the wide variety of potential physical, software,

Odisha State Open University Page 21

 and network vulnerabilities. Instead, the goal is to detect active attacks
and to recover from any disruption or delays caused by them. If the
detection has a deterrent effect, it may also contribute to prevention.
5. The attacks on confidentiality include the following:
· Snooping refers to unauthorized access to or interception of data.
· Traffic analysis refers to obtaining some other type of information by
monitoring online traffic.
6. The attacks on Integrity include the following
· Modification means that the attacker intercepts the message and changes it.
· Masquerading or spoofing happens when the attacker impersonates somebody
else.
· Replaying means the attacker obtains a copy of a message sent by a user and
later tries to replay it.
· Repudiation means that sender of the message might later deny that she has sent
the message; the receiver of the message might later deny that he has
received the message.
7. The attacks on availability include the following.
· Denial of Service (DoS) is a very common attack. It may slow down
or totally interrupt the service of a system.

1.15 ANSWER TO SELF-ASSESSMENT QUESTIONS-2

1: Information security is both complex and challenging. Some of the reasons

are as follows:
(i) The major requirements for security services can be confidentiality,
authentication, nonrepudiation, or integrity. But the mechanisms used to meet
those requirements can be quite complex.
(ii) In developing a particular security mechanism or algorithm, one must always
consider potential attacks on those security features.
(iii) Typically, a security mechanism is complex, and it is not obvious from the
statement of a particular requirement that such elaborate measures are needed.
(iv) Having designed various security mechanisms, it is necessary to decide where
to use them. This is true both in terms of physical placement (e.g., at what
points in a network are certain security mechanisms needed) and in a logical
sense (e.g., at what layer or layers of an architecture such as TCP/IP

Page 22 Odisha State Open University

 [Transmission Control Protocol/Internet Protocol] should mechanisms be
placed).
(v) Security mechanisms typically involve more than a particular algorithm or
protocol. They also require that participants be in possession of some secret
information (e.g., an encryption key), which raises questions about the
creation, distribution, and protection of that secret information.
(vi) The attacker has is that he or she need only find a single weakness, while the
designer must find and eliminate all weaknesses to achieve perfect security.
(vii) There is a natural tendency on the part of users and system managers to
perceive little benefit from security investment until a security failure
occurs.
(viii) Security requires regular, even constant, monitoring, and this is difficult in
today's short-term, overloaded environment.
(ix) Security is still too often an afterthought to be incorporated into a system
after the design is complete rather than being an integral part of the design
process.
(x) Many users and even security administrators view strong security as an
impediment to efficient and user-friendly operation of an information system
or use of information.
1. Passive attacks are in the nature of eavesdropping on, or monitoring of,
transmission, the goal of the opponent is to obtain information that is being
transmitted. Two types of passive attacks are release of massage contents and
traffic analysis. The release of message contents is easily understood.
A telephone conversation, an electronic mail message, and a transferred file may
contain sensitive or confidential information. We would like to prevent an
opponent from learning the contents of these transmissions.
A second type of passive attack, traffic analysis, is subtler. 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 for masking contents is encryption. If we had encryption
protection in place, an opponent might still be able to observe the pattern of these
message .the opponent could determine the location and identity of
communicating hosts and could observe the frequency and length of messages
being exchanged .This information might be useful in guessing the nature of the
communication that was taking place . Passive attacks are very difficult to detect

Odisha State Open University Page 23

 because they do not involve any alteration of the data.
Typically, the message traffic is not sent and received in an apparently normal fashion
and the sender nor receiver is aware that a third party has read the message or
observed the traffic pattern.
However, it is feasible to prevent the success of these attacks, usually by means of
encryption. Thus, the emphasis in dealing with passive attacks is on prevention rather
than detection.
2. There are five security services: data confidentiality, data integrity,
authentication, on-repudiation, and access control.
· Data confidentiality is to protect data from disclosure attack.
· Data integrity is to protect data from modification, insertion, deletion, and
replaying.
· Authentication means to identify and authenticate the party at the other end of the
line.
· Nonrepudiation protects against repudiation by either the sender or the receiver
of the data.
· Access control provides protection against unauthorized access to data.

1.16 REFERENCES AND SUGGESTED READINGS

1. William Stallings, “Cryptography and Network Security-Principles and

Practices”, Prentice-Hall of India.

2. Charles P.Pfleeger, Shari Lawrence Pfleeger, even Shah, “Security in

Computing”, Pearson Education

3. Bernad Menezes, “Network Security & Cryptography”, CENGAGE

Learning.
Atul Kahate, “Cryptography and Network Security”, TMH Publishing
Company Limited

Page 24 Odisha State Open University

UNIT-2
INTRODUCTION TO CRYPTOGRAPHY

Unit Structure
2.0 Introduction
2.1 Learning Objectives
2.2 Basic Terminologies
2.3 Cryptographic Systems
2.4 Cryptanalysis
2.5 Steganography
2.6 Network Security Model
2.7 Types of Cryptography
2.8 Symmetric key Cryptography
2.8.1 Unconditionally secure vs. computationally secured Encryption
2.8.2 Block Cipher Vs. Stream Cipher
2.9 Classical Encryption Techniques
2.9.1 Substitution Techniques
2.9.1.1 Monoalphabetic Cipher
2.9.1.2 Polyalphabetic Cipher
2.9.2 Transposition Techniques
2.10 Self-Assessment Questions
2.11 Key terms & Concepts
2.12 Answer to Self-Assessment Questions
2.13 References and Suggested Readings

Odisha State Open University Page 25

 2.0 INTRODUCTION

Cryptography, which comes from the Greek words kryptos, meaning "hidden," and
graphein, meaning "to write," is the process of making and using codes to secure the
transmission of information. Cryptanalysis is the process of obtaining the original
message (called the plaintext) from an encrypted message (called the ciphertext)
without knowing the algorithms and keys used to perform the encryption.

The science of encryption and decryption is known as cryptology. Cryptology

encompasses both cryptography and cryptanalysis.

Encryption is the process of converting an original message into a form that is

unreadable to unauthorized individuals-that is; to anyone without the tools to
convert the encrypted message back to its original format.

Decryption is the process of converting the cipher text into a message that conveys
readily understood meaning.
In order to understand cryptography and its uses, you must be familiar with a number
of key terms that are used in cryptography.
In this unit, we will discuss the basic cryptography concepts and its related
terminologies used in the area of Cryptography.

2.1 LEARNING OBJECTIVES

After studying this unit, you should be able to:

· Understand the basic terms used in cryptography.
· Understand the principles of cryptography and cryptanalysis.
· Describe different types of cryptanalytic attacks.
· Different types of cryptography
· Discuss the network security model
· Explain different classical encryption techniques.

2.2 BASIC TERMINOLOGIES

Cryptography
The art or science encompassing the principles and methods of transforming an

Page 26 Odisha State Open University

intelligible message into one that is unintelligible, and then retransforming that
message back to its original form is called cryptography.
Plaintext: The original intelligible message.
Ciphertext: The transformed message.
Cipher: An algorithm for transforming an intelligible message into one that is
unintelligible by transposition and/or substitution methods
Key: Some critical information used by the cipher, known only to the sender&
receiver
Encipher (encode): The process of converting plaintext to cipher text using a
cipher and a key.
Decipher (decode): the process of converting cipher text back into plaintext using
a cipher and a key.
Cryptanalysis: The study of principles and methods of transforming an
unintelligible message back into an intelligible message without knowledge of the
key. It is also called code breaking.
Cryptology: It includes both cryptography and cryptanalysis
Code: An algorithm for transforming an intelligible message into an
unintelligible one using a code-book.

2.3 CRYPTOGRAPHIC SYSTEMS

Cryptographic systems are generally classified along three independent

dimensions:
(i) Type of operations used for transforming plain text to cipher text
All the encryption algorithms are based on two general principles: substitution, in
which each element in the plaintext is mapped into another element, and
transposition, in which elements in the plaintext are rearranged.
(ii) The number of keys used
If the sender and receiver uses same key then it is said to be symmetric key (or)
single key (or) conventional encryption. If the sender and receiver use different
keys then it is said to be public key encryption.
(iii) The way in which the plain text is processed
A block cipher processes the input and block of elements at a time, producing
output block for each input block.
A stream cipher processes the input elements continuously, producing output
element one at a time, as it goes along.

Odisha State Open University Page 27

 2.4 CRYPTANALYSIS

The process of attempting to discover the plaintext or the key or both is known as
cryptanalysis. The strategy used by the cryptanalyst depends on the nature of the
encryption scheme and the information available to the cryptanalyst.
A cryptanalyst can do any or all of six different things:
1. Attempt to break a single message
2. Attempt to recognize patterns in encrypted messages, to be able to break
subsequent ones by applying a straightforward decryption algorithm
3. Attempt to infer some meaning without even breaking the encryption, such as
noticing an unusual frequency of communication or determining something by
whether the communication was short or long
4. Attempt to deduce the key, in order to break subsequent messages easily
5. Attempt to find weaknesses in the implementation or environment of use of
encryption
6. Attempt to find general weaknesses in an encryption algorithm, without
necessarily having intercepted any messages
2.4.1 Types of Cryptanalytic Attacks
There are various types of cryptanalytic attacks based on the amount of
information known to the cryptanalyst.
· Cipher text only – A copy of cipher text alone is known to the cryptanalyst.
· Known plaintext – The cryptanalyst has a copy of the cipher text and the
corresponding plaintext.
· Chosen plaintext – The cryptanalysts gains temporary access to the encryption
machine. They cannot open it to find the key, however; they can encrypt a large
number of suitably chosen plaintexts and try to use the resulting cipher texts to
deduce the key.
· Chosen cipher text – The cryptanalyst obtains temporary access to the
decryption machine, uses it to decrypt several string of symbols, and tries to use the
results to deduce the key.

2.4.2 Brute-force attack

The attacker tries every possible key on a piece of cipher text until an intelligible
translation into plaintext is obtained. On an average, half of all possible keys must be
tried to achieve success. If either type of attack succeeds in deducing the key, the
effect is catastrophic: All future and past messages encrypted with that key are
compromised.

Page 28 Odisha State Open University

2.5 STEGANOGRAPHY

A plaintext message may be hidden in any one of the two ways. The methods of
steganography conceal the existence of the message, whereas the
methods of cryptography render the message unintelligible to outsiders by
various transformations of the text. A simple form of steganography, but one that is
time consuming to construct is one in which an arrangement of words or letters
within an apparently innocuous text spells out the real message. Let us consider the
following examples.
(I) The sequence of first letters of each word of the overall message spells out the
real (Hidden) message.
(ii) Subset of the words of the overall message is used to convey the hidden
message. Various other techniques have been used historically, some of them are:
Character marking – selected letters of printed or typewritten text are overwritten
in pencil. The marks are ordinarily not visible unless the paper is held to an angle to
bright light.
Invisible ink – a number of substances can be used for writing but leave no visible
trace until heat or some chemical is applied to the paper.
Pin punctures – small pin punctures on selected letters are ordinarily not visible
unless the paper is held in front of the light.
Type written correction ribbon – used between the lines typed with a black
ribbon, the results of typing with the correction tape are visible only under a strong
light.
Drawbacks of Steganography
It requires a lot of overhead to hide a relatively few bits of information. Once the
system is discovered, it becomes virtually worthless.

2.6 NETWORK SECURITY MODEL

A model for network security is shown in the following figure. A message is to be

transferred from one party to another across some sort of Internet service. 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 the Internet from source to destination and by the cooperative use of

Odisha State Open University Page 29

 communication protocols (e.g., TCP/IP) by the two principals. Security aspects
come into 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 two
components:
One is a security-related transformation on the information to be sent. Examples
include the encryption of the message, which scrambles the message so that it is
unreadable by the opponent, and the other is the addition of a code based on the
contents of the message, which can be used to verify the identity of the sender.
Some secret information 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
reception.
A trusted third party 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. Otherwise a third party may be
needed to arbitrate disputes between the two principals concerning the authenticity
of a message transmission.

Fig: Network Security Model

Page 30 Odisha State Open University

This general model shows that there are four basic tasks in designing a particular
security service:
1. Design an algorithm for performing the security-related transformation. The
algorithm should be such that an opponent cannot defeat its purpose.
2. Generate the secret information to be used with the algorithm.
3. Develop methods for the distribution and sharing of the secret information.
4. Specify a protocol to be used by the two principals that makes use of the
security algorithm and the secret information to achieve a particular security
service.

2.7 TYPES OF CRYPTOGRAPHY

Based on the security models different cryptographic algorithms have been

developed to prevent and defend attacks. Based on the type of algorithms
cryptographic systems are categories in to two categories.
1. Symmetric key Cryptography
In symmetric key Cryptography, the encryption and decryption keys are known
both to sender and receiver. The encryption key is shared and the decryption key is
easily calculated from it. In many cases, the encryption and decryption keys are the
same.
2. Public key Cryptography
In public key cryptography, encryption key is made public, but it is
computationally infeasible to find the decryption key without the information
known to the receiver.

2.8 SYMMETRIC KEY CRYPTOGRAPHY

A symmetric encryption scheme has five ingredients:

Plaintext: 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 plaintext.
Secret key: The secret key is also input to the encryption algorithm. The key is a
value independent of the plaintext and of the algorithm. The algorithm will
produce a different output depending on the specific key being used at the time.

Odisha State Open University Page 31

 The exact substitutions and transformations performed by the algorithm depend on
the key.
Ciphertext: This is the scrambled message produced as output. It depends on the
plaintext and the secret key. For a given message, two different keys will produce
two different ciphertexts. The ciphertext is an apparently random stream of data and,
as it stands, is unintelligible.
Decryption algorithm: This is essentially the encryption algorithm run in reverse.
It takes the ciphertext and the secret key and produces the original plaintext.
There are two requirements for secure use of conventional encryption:
1. We need a strong encryption algorithm. At a minimum, we would like the
algorithm to be such that an opponent who knows the algorithm and has access to one
or more ciphertexts would be unable to decipher the ciphertext or figure out the key.
This requirement is usually stated in a stronger form: The opponent should be unable
to decrypt ciphertext or discover the key even if he or she is in possession of a number
of ciphertexts together with the plaintext that produced each ciphertext.
2. Sender and receiver must have obtained copies of the secret key in a secure
fashion and must keep the key secure. If someone can discover the key and knows the
algorithm, all communication using this key is readable.
We assume that it is impractical to decrypt a message on the basis of the ciphertext
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.

Fig.: Symmetric key Cryptography

Page 32 Odisha State Open University

This feature of symmetric encryption is what makes it feasible for widespread use.
The fact that the algorithm need not be kept secret means that manufacturers can
and have developed low-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.
Let us take a closer look at the essential elements of a symmetric encryption
scheme, using the following figure.
A source produces a message in plaintext, X = [X1, X2... XM]. The M elements of X
are letters in some finite alphabet. Traditionally, the alphabet usually consisted of
the 26 capital letters. Now days, the binary alphabet {0, 1} is typically used. For
encryption, a key of the form K = [K1, K2... KJ] is generated. If the key is generated
at the message source, then it must also be provided to the destination by means of
some secure channel. Alternatively, a third party could generate the key and
securely deliver it to both source and destination.
With the message X and the encryption key K as input, the encryption algorithm
forms the cipher text Y = [Y1, Y2... YN]. We can write this as
Y = E (K, X)
This notation indicates that Y is produced by using encryption algorithm E as a
function of the plaintext X, with the specific function determined by the value
of the key K.
The intended receiver, in possession of the key, is able to invert the
transformation:
X = D (K, Y).

Fig.: Essential elements of a symmetric encryption

Odisha State Open University Page 33

 An opponent, observing Y but not having access to K or X, may attempt to recover X
or K or both X and K. It is assumed that the opponent knows the encryption (E) and
decryption (D) algorithms. If the opponent is interested in only this particular
message, then the focus of the effort is to recover X by generating a plaintext estimate

2.8.1 Unconditionally secure vs. computationally secure Encryption

Unconditionally secure Encryption:
An encryption scheme is unconditionally secure if the ciphertext generated by the
scheme does not contain enough information to determine uniquely the
corresponding plaintext, no matter how much ciphertext is available. That is, no
matter how much time an opponent has, it is impossible for him or her to decrypt the
ciphertext, simply because the required information is not there. There is no
encryption algorithm that is unconditionally secure. Therefore, all that the users of an
encryption algorithm can strive for is an algorithm that meets one or both of the
following criteria:
· The cost of breaking the cipher exceeds the value of the encrypted information.
· The time required to break the cipher exceeds the useful lifetime of the
information.
Computationally secure Encryption
An encryption scheme is said to be computationally secure if either of the foregoing
two criteria are met. The rub is that it is very difficult to estimate the amount of effort
required to cryptanalyze ciphertext successfully.
1.8.2 Block Cipher Vs. Stream Cipher
Now we will discuss the method in which the plain text is converted into cipher text.
In some methods, plain text is treated as numerous units or blocks and then it is
converted into cipher text. But in some methods, plain text is divided into bits and
these bits individually are given as input to the method which converts each single bit
to the cipher text.
Therefore, there are two cipher methods (Block Cipher and Stream Cipher) in which
plain text is given as input in order to convert them to their corresponding cipher text
using any of these ciphering techniquees.
Block Cipher
Block Cipher, as the name suggests, takes input (i.e. plain text) and divides the plain
text into number of units or blocks. After receiving input, plain text as a unit or block
is encrypted with the key and converts it to a cipher text.
Advantages of Block Cipher:

Page 34 Odisha State Open University

• It is faster than stream cipher.
• If any block contains any transmission error then it will not have effect on other
blocks.
• It is not sufficient in hardware but may be used to connect keyboard to CPU
(central process unit)
• Block ciphers can be easier to implement in software, because there is no bit
manipulation like in stream cipher which is time consuming process and treats data
in computer-sized blocks
• Block Cipher is more suitable in trading applications.
• Short blocks at the end of a message can also be added with blank or zero status.
Disadvantages of Block Cipher:
• If two same unit or blocks of plaintext is there, then the cipher produces same
cipher text for units or blocks.
• It is easy to insert or delete units/blocks.
• Block encryption may be more susceptible to cryptanalysis attack as compared
to stream cipher as block cipher provides strong hints to attacker because two same
units produce same cipher text.
• Block encryption is more susceptible to replay as compare to stream
encryption.
Stream Cipher
Stream Cipher takes input (i.e. plain text) and divide this plain text into number of
bits (combination of such bits is plain text). After receiving single bit which
represents as a part of plain text is encrypted with the key and converts it to a cipher
text.
Advantages of Stream Cipher:
• Stream cipher is suitable for hardware implementation as only encryption and
decryption data one bit at a time.
• Stream cipher is less susceptible to cryptanalysis than block cipher as block
cipher provides strong hints to attacker because two same units produce same
cipher text.
• Stream cipher is less vulnerable to insertion or deletion of units.
• This cipher can be more easily analyzed mathematically.
• It is more suitable for military applications.
Disadvantage of Stream Cipher:
• If during transmission, any bit is lost or become erroneous, then it is difficult to

Odisha State Open University Page 35

 re-arrange and collect all the converted cipher text which in return may result in re-
transmission of the entire plain text.
• It is slower than block but can be configured to make faster by implemented in
special purpose hardware capable of encryption several million bits for second.
• It is not suitable for the software.

2.9 CLASSICAL ENCRYPTION TECHNIQUES

The two basic building blocks of Classical encryption techniques are substitution
and transposition.
2.9.1 Substitution Techniques
A substitution technique is one in which the letters of plaintext are replaced by other
letters or by numbers or symbols to obtain the cipher text.
If the plaintext is viewed as a sequence of bits, then substitution involves replacing
plaintext bit patterns with ciphertext bit patterns.
Caesar Cipher
The earliest known, and the simplest, use of a substitution cipher was by Julius
Caesar. The Caesar cipher involves replacing each letter of the alphabet with the
letter standing three places further down the alphabet. Let us consider the following
example.
Plaintext: meet me after the toga party
Cipher: PHHW PH DIWHU WKH WRJD SDUWB
Note: The alphabet is wrapped around, so the letter following Z is A.
We can define the transformation by listing all possibilities, as follows:
Plaintext: 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
Ciphertext: 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 numerical equivalent to each letter:

A B C D E F G H I J K L M

0 1 2 3 4 5 6 7 8 9 10 11 12

N O P Q R S T U V W X Y Z

13 14 15 16 17 18 19 20 21 22 23 24 25

Page 36 Odisha State Open University

Then the algorithm can be expressed as follows. For each plaintext letter: p,
substitute the cipher text letter: C
C = E (3, p) = (p + 3) mod 26
Encryption:
A shift may be of any amount, so that the general Caesar algorithm is
C = E (k, p) = (p + k) mod 26
Where, k takes on a value in the range 1 to 25.
Decryption:
The decryption algorithm for the Caesar Cipher is simply
p = D (k, C) = (C - k) mod 26
Limitations of the Caesar Cipher
If it is known that a given ciphertext is a Caesar cipher, then a brute-force
cryptanalysis is easily performed: simply try all the 25 possible keys.

· Note: Three important characteristics of this problem enabled us to use a brute

force
Cryptanalysis of Caesar Cipher:
1. The encryption and decryption algorithms are known.
2. There are only 25 keys to try.
3. The language of the plaintext is known and easily recognizable.2.9.1.1
Monoalphabetic Ciphers
A monoalphabetic cipher uses a fixed substitution over the entire message. It is an
improvement over Caesar Cipher. It eliminates the limitations of Caesar Cipher.
The Caesar cipher is far from secure with only 25 possible keys, in which a simple
brute force cryptanalysis is possible.
A dramatic increase in the key space can be achieved by allowing an arbitrary
substitution. Before proceeding, we define the term permutation.
A permutation of a finite set of elements is an ordered sequence of all the elements
of S, with each element appearing exactly once.
For example, if ,S={a,b,c}, there are six permutations of S:
abc, acb, bac, bca, cab, cba
In general, there are n! permutations of a set of n elements, because the first element
can be chosen in one of n ways, the second in (n-1)ways, the third in (n-2)ways, and
so on.
Recall the assignment for the Caesar cipher:
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

Odisha State Open University Page 37

 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
If, instead, the “cipher” line can be any permutation of the 26 alphabetic characters,
then there are 26! or greater than 4×1026 keys.

Play fair Cipher

The best-known multiple-letter encryption cipher is the Playfair, which treats
digrams in the plaintext as single units and translates these units into ciphertext
digrams. The Playfair algorithm is based on the use of a 5 × 5 matrix of letters
constructed using a keyword.

Example: Let us consider the example solved by Lord Peter Wimsey in Dorothy
Sayers's Have His Carcase:
M O N A R
C H Y B D
E F G I/J K
L P Q S T
U V W X Z

In this case, the keyword is monarchy. The matrix is constructed by filling in the
letters of the keyword (minus duplicates) from left to right and from top to bottom,
and then filling in the remainder of the matrix with the remaining letters in alphabetic
order. The letters I and J count as one letter. Plaintext is encrypted two letters at a
time, according to the following rules:
1. Repeating plaintext letters that are in the same pair are separated with a filler
letter, such as x, so that balloon would be treated as ba lx lo on.
2. Two plaintext letters that fall in the same row of the matrix are each replaced
by the letter to the right, with the first element of the row circularly following
the last. For example, ar is encrypted as RM.
3. Two plaintext letters that fall in the same column are each replaced by the
letter beneath, with the top element of the column circularly following the
last. For example, mu is encrypted as CM.
4. Otherwise, each plaintext letter in a pair is replaced by the letter that lies in its
own row and the column occupied by the other plaintext letter. Thus, hs
become BP and ea becomes IM (or JM, as the encipherer wishes).
The Playfair cipher is a great advance over simple monoalphabetic ciphers.
For one thing, whereas there are only 26 letters, there are 26 × 26 = 676 digrams, so
that identification of individual digrams is more difficult. Furthermore, the relative
frequencies of individual letters exhibit a much greater range than that of digrams,

Page 38 Odisha State Open University

making frequency analysis much more difficult. For these reasons, the Play air
cipher was for a long time considered unbreakable. It was used as the standard field
system by the British Army in World War I and still enjoyed considerable use by the
U.S. Army and other Allied forces during World War II.
Limitations of Playfair Cipher:
Despite this level of confidence in its security, the Play air cipher is relatively easy
to break, because it still leaves much of the structure of the plaintext language
intact. A few hundred letters of ciphertext are generally sufficient.
2.9.1.2 Polyalphabetic Ciphers
Another way to improve on the simple monoalphabetic technique is to use different
monoalphabetic substitutions as one proceeds through the plaintext message. The
general name for this approach is polyalphabetic cipher. All the techniques have the
following features in common. A set of related monoalphabetic substitution rules
are used A key determines which particular rule is chosen for a given
transformation.
Vigenere Cipher:
In this scheme, the set of related monoalphabetic substitution rules consisting of 26
Caesar ciphers with shifts of 0 through 25. Each cipher is denoted by a key letter.
e.g., Caesar cipher with a shift of 3 is denoted by the key value 'd‟ (since a=0, b=1,
c=2 and so on).
To aid in understanding the scheme, a matrix known as vigenere tableau is
constructed each of the 26 ciphers is laid out horizontally, with the key letter for
each cipher to its left. A normal alphabet for the plaintext runs across the top. Refer
the following Vigenere tableau.
The process of Encryption is simple: Given a key letter X and a plaintext letter y, the
cipher text is at the intersection of the row labelled x and the column labelled y; in
this case, the cipher text is V.
To encrypt a message, a key is needed that is as long as the message. Usually, the
key is a repeating keyword.

Odisha State Open University Page 39

 Table: Vigenere tableau

Example: Let us consider the following.

Key = d e c e p t i v e d e c e p t i v e d e c e p t i v e
PT = w e a r e d i s c o v e r e d s a v e y o u r s e l f
CT = ZICVTWQNGRZGVTWAVZHCQYGLMGJ

Decryption is equally simple. The key letter again identifies the row. The position of
the cipher text letter in that row determines the column, and the plaintext letter is at
the top of that column.
Advantages of Vigenere Cipher
· There are multiple cipher text letters for each plaintext letter, so it is almost
secured.
Disadvantages of Vigenere Cipher
· The key is periodically repeated
· Data can be detected from the key length
One Time Pad Cipher
It is an unbreakable cryptosystem. It represents the message as a sequence of 0s
and 1s.This can be accomplished by writing all numbers in binary, for example, or

Page 40 Odisha State Open University

by using ASCII. The key is a random sequence of 0's and 1's of same length as
the message. Once a key is used, it is discarded and never used again. The
system can be expressed as follows:
Ci = Pi Ki
th
Ci - i binary digit of cipher text
th
Pi - i binary digit of plaintext
th
Ki - i binary digit of key

Exclusive OR operation
Thus the cipher text is generated by performing the bitwise XOR of the plaintext
and the key.
Decryption uses the same key. Because of the properties of XOR, decryption
simply involves the same bitwise operation:
Pi = Ci Ki
e.g., plaintext = 0 0 1 0 1 0 0 1
Key = 1 0 1 0 1 1 0 0
Ciphertext = 1 0 0 0 0 1 0 1
Advantage:
· Encryption method is completely unbreakable for a ciphertext only attack.
Disadvantages:
· It requires a very long key which is expensive to produce and expensive to
transmit.
· Once a key is used, it is dangerous to reuse it for a second message; any
knowledge on the first message would give knowledge of the second.
2.9.2 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.
Rail fence techniques:
Rail fence is simplest of such cipher, in which the plaintext is written down as a
sequence of diagonals and then read off as a sequence of rows.
Plaintext = meet at the school house
To encipher this message with a rail fence of depth 2, we write the message as
follows:
meatecolos
etthshohue

Odisha State Open University Page 41

 The encrypted message is MEATECOLOSETTHSHOHUE
Row Transposition Cipher
A more complex scheme is to write the message in a rectangle, row by row, and read
the message off, column by column, but permute the order of the columns. The order
of columns then becomes the key of the algorithm.
e.g., plaintext = meet at the school house
Key = 4 3 1 2 5 6 7
Plain Text = m e e t a t t
h e s c h o o
l h o u s e

The cipher text is: esotcueehmhlahstoeto

Strength of Row Transposition Cipher
A pure transposition cipher is easily recognized because it has the same letter
frequencies as the original plaintext. The transposition cipher can be made
significantly more secure by performing more than one stage of transposition. The
result is more complex permutation that is not easily reconstructed.

2.10 SELFASSESSMENT QUESTIONS

1. Define cryptanalysis .List the types of cryptanalytic attacks.

………………………………………………………………………………………
………………………………………………………………………………………
………………………………………………………………………………………
…………................................................................................................................…
2. Differentiate symmetric and asymmetric key Cryptography.
………………………………………………………………………………………
………………………………………………………………………………………
………………………………………………………………………………………
……................................................................................................................………
3. Differentiate unconditionally secured and computationally secured
algorithm.
………………………………………………………………………………………
………………………………………………………………………………………
………………………………………………………………………………………
……….................................................................................................................…..

Page 42 Odisha State Open University

4. Specify the components of secret key cryptography.
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
………………..........…………………………………………………………..…
5. Compare Substitution and Transposition techniques.
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………........…
6. Differentiate between Block cipher and Stream Cipher.
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………….......
1.11 KEY TERMS & CONCEPTS
· Cryptology is the study of cryptography and cryptanalysis. Cryptology
encompasses both cryptography and cryptanalysis.
· Cryptography is the study of principles and methods of transforming a
message into an unintelligible form and then retransforming that message back to
its original form.
· Cryptanalysis is the process of attempting to discover the plaintext or the key
or both without knowledge of the key. It is also called code breaking
· Steganography is a technique to conceal the existence of the message.
· Transposition cipher is an encryption technique which is achieved by
performing some sort of permutation on the plaintext letters.
· A block cipher processes the input one block of elements at a time, producing
an output block for each input block
· A stream cipher processes the input elements continuously, producing output
one element at a time, as it goes along.

2.12 ANSWER TO SELFASSESSMENT QUESTIONS

1. Cryptanalysis is a process of attempting to discover the key or plaintext or both.

The types of cryptanalytic attacks are:

Odisha State Open University Page 43

 (i) Cipher text only
(ii) Known plaintext
(iii) Chosen plaintext
(iv) Chosen cipher text
(v) Chosen text
2. Difference between symmetric and asymmetric key cryptography are as follows:
Symmetric Cryptography uses single key to encrypt and decrypt data whereas
Asymmetric Cryptography uses public key to encrypt the data and private key to
decrypt it.
Symmetric key cryptography is much faster than asymmetric key encryption.
Symmetric key cryptography does not require a lot of computer resources when
compared to public key encryption which uses up more computer resources.
In Symmetric Cryptography, secret key exchange is a problem. But in asymmetric
cryptography, there is no such key exchange problem.
In Symmetric Cryptography, origin and authenticity of message cannot be
guaranteed whereas Asymmetric Cryptography provides method for message
authentication, detection of tampering, non-repudiation.
Symmetric Cryptography prevents widespread message security compromise but in
Asymmetric Cryptography, widespread security compromise is possible.
• Examples of Symmetric Cryptography include DES, AES and examples of
Asymmetric Cryptography are RSA, ECC.
2. An Encryption algorithm is unconditionally secured means:
(i) If the cipher text generated by the encryption scheme doesn't contain enough
information to determine corresponding plaintext.
An Encryption Algorithm is computationally secured means:
(i) The cost of breaking the cipher exceeds the value of enough information.
(ii) Time required to break the cipher exceed the useful lifetime of information.
4. The five components of secret key cryptography are:
(i) Plaintext
(ii) Encryption algorithm
(iii) Secret key
(iv) Cipher text
(v) Decryption algorithm
5. The two encryption techniques are different in the following ways.

Page 44 Odisha State Open University

 Substitution Transposition
· A substitution techniques is · It means, different kind of
one in which the letters of mapping is achieved by
plaintext are replaced by other performing some sort of
letter or by number or permutation on the plaintext
symbols. letters.
· Example: Caesar cipher. · Example: DES, AES

6. Following are the advantages and disadvantages of Block and Stream Cipher.
Advantages of Block Cipher:
• It is faster than stream cipher.
• If any block contains any transmission error then it will not have effect on other
blocks.
• It is not sufficient in hardware but may be used to connect keyboard to CPU
(central process unit)
• Block ciphers can be easier to implement in software, because there is no bit
manipulation like in stream cipher which is time consuming process and treats data
in computer-sized blocks
• Block Cipher is more suitable in trading applications.
• Short blocks at the end of a message can also be added with blank or zero status.
Disadvantages of Block Cipher:
• If two same unit or blocks of plaintext is there, then the cipher produces same
cipher text for units or blocks.
• It is easy to insert or delete units/blocks.
• Block encryption may be more susceptible to cryptanalysis attack as compared
to stream cipher as block cipher provides strong hints to attacker because two same
units produce same cipher text.
• Block encryption is more susceptible to replay as compare to stream
encryption.
Advantages of Stream Cipher
• Stream cipher is suitable for hardware implementation as only encryption and
decryption data one bit at a time.
• Stream cipher is less susceptible to cryptanalysis than block cipher as block
cipher provides strong hints to attacker because two same units produce same
cipher text.
• Stream cipher is less vulnerable to insertion or deletion of units.

Odisha State Open University Page 45

 • This cipher can be more easily analyzed mathematically.
• It is more suitable for military applications.
• It is less useful for attackers as same plain text is encrypted but in single
individual bits and not in units.
Disadvantage of Stream Cipher:
• If during transmission, any bit is lost or become erroneous, then it is difficult to
re-arrange and collect all the converted cipher text which in return may result in re-
transmission of the entire plain text.
• It is slower than block but can be configured to make faster by implemented in
special purpose hardware capable of encryption several million bits for second.
• It is not suitable for the software.

2.13 REFERENCES AND SUGGESTED READINGS

1. William Stallings, “Cryptography and Network Security-Principles and

Practices”, Prentice-Hall of India.
2. Charles P.Pfleeger, Shari Lawrence Pfleeger, even Shah, “Security in
Computing”, Pearson Education
3. Bernad Menezes, “Network Security & Cryptography”, CENGAGE Learning.
4. Atul Kahate, “Cryptography and Network Security”, TMH Publishing
Company Limited.

Page 46 Odisha State Open University

UNIT-3
CRYTOGRAPHIC ALGORITHMS

3.0 Introduction
3.1 Learning Objectives
3.2 Cryptographic Algorithms
3.2.1 Hash Algorithm
3.2.2 Message Digest (MD)
3.2.3 Message Digest 4
3.2.4 Message Digest 5 (MD5)
3.2.5 Secure Hash algorithm (SHA)
3.2.6 Whirlpool
3.2.7 RACE Integrity Primitives Evaluation Message Digest (RIPEMD)
3.3 Password Hashes
3.4 Symmetric Cryptographic Algorithms
3.4.1 Understanding Symmetric Algorithms
3.4.2 Block Cipher
3.4.3 Data Encryption System (DES)
3.4.4 Triple Data Encryption Standard (3DES)
3.4.5 Advance Encryption Standard (AES)
3.4.6 Other Algorithms
3.5 Asymmetric Cryptographic Algorithms
3.6 RSA Algorithm
3.6.1 Elliptic curve Cryptography (ECC)
3.7 Using Cryptography
3.7.1 Encryption through Software
3.7.2 File and File System Cryptography
3.7.3 Pretty Good Privacy (PGP/GPG)
3.8 Self-Assessment Questions
3.9 Key Terms and Concepts
3.10 Answer to Self-Assessment Questions.
3.11 References and Further Readings

Odisha State Open University Page 47

 3.0 INTRODUCTION

We have discussed in the previous unit that cryptography is an important means of

protecting information from attackers.
Cryptography is the science of transforming information into a secure form so that it
can be transmitted or stored and unauthorized persons cannot access it. Whereas
Cryptography scrambles a message so that it cannot be viewed, steganography hides
the existence of data. What appears to be a harmless image can contain hidden data,
usually some types of messages, embedded within the image. In this unit we will
discuss about different algorithms designed for encryption and decryption in both
symmetric and asymmetric encryption algorithms.

3.1 LEARNING OBJECTIVES

· After going through this unit, you will be able to understand:

· What is different cryptographic algorithm?
· What is Hashing, Message Digest?
· What is the difference between Symmetric and Asymmetric key
Cryptographic algorithm?
· How Cryptography is used in secure applications?

3.2 CRYTOGRAPHIC ALGORITHMS.

There are three categories of cryptographic algorithms. These are known as hash
algorithms, symmetric encryption algorithms, and asymmetric encryption
algorithms.
3.2.1 Hash Algorithm
The most basic type of cryptographic algorithm is a hash algorithm. Hashing is a
process for creating a unique digital fingerprint for a set of data. This fingerprint,
called a hash (sometimes called a one-way hash or digest) represents the contents.
Although hashing is considered a cryptographic algorithm, its purpose is not to
create a ciphertext that can later be decrypted.
Hashing is primarily used for comparison purposes. A hash that is created from a set
of data cannot be reversed. For example, if 12,345 is multiplied by 143, the result is
1,765,335. If the number 1,765,335 was given to a user, and the user was asked to

Page 48 Odisha State Open University

determine the two original numbers to create1, 765, 335, it would be virtually
impossible to work backward and derive the original numbers. This is because
there are too many mathematical possibilities.
Hashing is similar in that is used to create a value, yet it is not possible to work
―backward to determine the original set of data. A practical example of hash
algorithm is used with some automated teller machine (ATM) cards. A bank
customer has a personal identification number (PIN) of 93542. This number is
hashed and the result is permanently stored on a magnetic stripe on the back of the
ATM card. When visiting an ATM, the customer is asked to insert the card and then
enter the PIN on a keypad. ATM takes the PIN entered and hashes it with the same
algorithm used to create the hash stored on the card. If the two values match, then
the user can access the ATM. Hashing with ATMs is shown below in the figure.

Fig: Hashing at an ATM

Hashing algorithm is considered secure if it has these characteristics:
Fixed size: A hash of a short set of data should produce the same size as a hash
of a long set of data. For example, a hash

i. of the single letter a is 86be7afa339d0fc7cfc785e72f578d33, while a hash

of 1 million occurrences of the letter a is 4a7f5723f954eba1216c9d8f6320431f,,
the same length.
ii. Unique: Two different sets of data cannot produce the same hash, which is
known as a collision. Changing a single letter in one data set should produce an
entirely different hash. For example, a hash of Today is Sunday
is8b9872b8ea83df7152ec0737d46bb951 while the hash of Today is Sunday
(changing the initial S to s) is 4ad5951de752ff7f579a86bfafc2c.
iii. Original: It should be impossible to produce a data set that has a desired or

Odisha State Open University Page 49

 predefined hash.
iv. Secure: The resulting hash cannot be reversed in order to determine the
original plaintext

Hashing is used to determine the integrity of a message or contents of a file. In this

case, the hash serves as a check to verify that the original contents have not been
changed. For example, when an e-mail message is created, a hash can also be created
based on the message contents. The message and the hash are transmitted to the
recipient, or the hash is posted where the reader can retrieve it. Upon receiving the
message, the same hash is generated again on the message. If the original hash is the
same as the new hash, then the message has not been altered. However, if an attacker
performs a man-in-middle attack to intercept and change the message, the hash
values will not match. Using hashing for protecting against man-in-the-middle
attacks as shown in figure below.

Fig: Man-in-the-middle attack defeated by Hashing

A variation that provides improved security is the Hashed Message Authentication
Code (HMAC). HMAC begins with a shared secret key that is in the possession of
both the sender and receiver. The sender creates a hash and then encrypts that hash
with the key before transmitting it with original data. The receiver uses their key to
decrypt the hash and then creates their own hash of the data, comparing the two
values. HMAC is widely used by Internet security protocols to verify the integrity of
transmitted data during secure communications. Hash values are often posted on
download sites in order to verify the integrity of files that can be downloaded.
Hashing can be used to verify the integrity of data. The protections provided by
hashing are given in the below table.
Table: Information protections by hashing cryptography

Page 50 Odisha State Open University

The Most common hash algorithms are Message Digest, Secure hash Algorithm.
Whirlpool, RIPEMD, and Password hashes.
3.2.2 Message Digest (MD)
One common hash algorithm is the Message Digest (MD) algorithm, which has
three versions. Message Digest 2 (MD2) takes plaintext of any length and creates
hash 128 bits long. MD2 begins by dividing the message into 128 bit sections. If the
message is less than 128 bit, data known as padding is added. For example, if a 10-
byte message is abcdefghij, MD2 would pad the message to make it
abcdefghij666666 to create a length of 16 bytes (128 bits). The padding is always
the number of bytes that must be added to create a length of16 bytes; in this
example, 6 is he padding because 6 more bytes had to be added to the 10 original
bytes. After padding, a 16-byte checksum is appended to the message. Then the
entire string is proces128 bit hash. MD2 was developed in 1989 and was optimized
to run on Intel-based computers that processed 8 bit at a time. MD2 is considered
too slow today and is rarely used.
3.2.3 Message Digest 4
(MD4) was developed in 1990 for computers that processed 32 bits at a time. Like
MD2, MD4 takes plaintext and creates a hash of128 bits. The plaintext message
itself is padded to a length of 512 bits instead of 128 bits. The plaintext message
itself is padded to a length of 512 bits instead of 128 bits as with MD2. Flaws in the
MD4 hash algorithm have prevented this MD from being widely accepted.
3.2.4 Message Digest 5 (MD5)
A revision of MD4, was created the following year and designed to address MD4's
weaknesses.
Like MD4, the length of a message is padded to 512 bits. The hash algorithm then
uses four variables of 32 bits each in a round-robin fashion to create a value that is
compressed to generate the hash. Weaknesses have been revealed in the
compression function that could lead to collisions, so some security experts
recommend that a more secure hash algorithm be used instead.
Note: The TCP/IP protocol Simple Network Management Protocol (SNMP)
version 3 default protocol is MD5.
3.2.5 Secure Hash algorithm (SHA)
A more secure hash than MD is the Secure Hash algorithm (SHA). Like MD, the
SHA is a family of hashes. The first version was SHA-0, yet due to a flaw it was
withdraw shortly after it was first released. It successor, SHA-1, is patterned after

Odisha State Open University Page 51

 MD4 and MD5, but creates a hash that is 160 bit in length instead of 128 bits. SHA
pads messages of fewer than 512 bits with zeros and an integer that describes the
original length of the message. The padded message is then run through the SHA
algorithm to produce the hash.
Note: SHA-1 was developed in 1993 by the U.S. National Security Agency (NSA)
and the National Institute of Standards and Technology (NIST). The other hashes are
known as SHA-2,
SHA-2 actually is composed of four variations, known as SHA-224, SHA-256,
SHA-384 and SHA-512 (the number following SHA indicates the length in bits of
the hash that is generated). SHA-2 is considered to be secure hash. Yet, there have
been no weaknesses identified with it.
Note: In 2007, an open competition for new SHA-3 hash was announced. Of the 51
entries that were accepted to Round 1 of the competition, only 14 were selected for
Round 2 (one of the entries rejected was a new MD6). In late 2010, five finalists were
announced for the final round 3, with the new standard expected to appear by 2012.
3.2.6 Whirlpool
Whirlpool is a relatively recent cryptographic hash function that has received
international recognition and adoption by standards organizations, including the
International Organization for Standardization (ISO). Named after the first galaxy
recognized to have a spiral structure, it creates a hash of 512 bits. Whirlpool is being
implemented in several new commercial cryptography applications.
TIP: According to creators, Whirlpool will not be patented and can be freely used for
any purpose.
3.2.7 RACE Integrity Primitives Evaluation Message Digest (RIPEMD)
Another hash was developed by the Research and Development and Advanced
Communications technologies (RACE) organization, which is affiliated with the
European Union (EU). RIPEMD stands for RACE Integrity Primitives Evaluation
Message Digest, which was designed after MD4.The primary design feature of
REPEMD is two different and independent parallel chains of computation, the
results of which are then combined at the end of the process. RIPEMD has several
versions and all based on the length of the hash created. RIPEMD-128 is a
replacement for the original RIPEMD and is faster than RIPEMD-160. RIPEMD-
256 and RIPEMD-320 reduce the risk of collisions, yet do not provide any higher
levels of security.

Page 52 Odisha State Open University

3.3 PASSWORD HASHES

Another use for hashes is in strong password. When a password for an account is
created, the password is hashed and stored. When a user enters her password to log
in, that password is likewise hashed and compared with the stored hashed version;
if the two hashes match, then the user is authorized. Microsoft Windows operating
systems hash password in two ways. The first is known as the LM (LAN Manager)
hash. The LM hash is in not actually a hash, because a hash is a mathematical
function used to fingerprint the data. The LM hash instead uses a cryptography
one-way function (OWF): instead of encrypting the password with another key, the
password itself is the key. The LM has is considered a very weak function for
storing password. First, the LM hash is not case sensitive, meaning that there is no
difference between uppercase (A) and lowercase
(a). This significantly reduces the character set that an attacker must use. Second,
the LM has splits all password into two7- character parts. If the original password is
fewer the 14 characters, it simply pads the parts; if it is longer, the extra characters
are dropped. This means that an attacker attempting to break an LM hash must only
break two 7-character passwords from a limited character set. To address the
security issues in the LM hash, Microsoft introduced the NTLM (New Technology
LAN Manager) hash. Unlike the LM hash, the NTLM has does not limit stored
password to two 7-character parts. In addition, it is case sensitive and has larger
character set of 65,535 characters. The original version of NTLM uses a weak
cryptographic function and does not support more recent cryptographic methods;
Microsoft recommends that it should not be used. The current version is NTLMv2
and uses HMAC with MD5. It is considered a much stronger hashing algorithm.
The salt, along with the number of “rounds” (iterations) used with the salt, is stored
along with the “salted” password hash.

3.4 SYMMETRIC CRYPTOGRAPHIC ALGORITHMS

The original cryptographic algorithms for encrypting and decrypting documents

are symmetric
Cryptographic algorithms. These include the Data Encryption Standard, Triple
Data Encryption Standard, Advanced Encryption Standard, and several other
algorithms.

Odisha State Open University Page 53

 3.4.1 Understanding Symmetric Algorithms
Symmetric cryptographic algorithms use the same shared single key to encrypt
and decrypt a document. Unlike hashing in which the hash is not intended to be
decrypted, symmetric algorithms are designed to encrypt and decrypt the cipher
text; a document encrypted with a symmetric cryptographic algorithm by Bob
will be decrypted when received by Alice. It is therefore essential that the key be
kept confidential, because if an attacker obtained the key, he could read all the
encrypted documents. For this reason, symmetric encryption is also called Private
Key cryptography. Symmetric encryption is illustrated in Figure below where
identical keys are used to encrypt and decrypt a document.

Fig: Symmetric (Private Key) Cryptography

Page 54 Odisha State Open University

Symmetric algorithms, like other types of cryptographic algorithms, can be
classified into two categories based on the amount of data that is processed at a
time.

The first category is known as a stream cipher. A stream cipher takes one
character and replaces it with one character, as shown in figure

Fig: Stream Cipher

Note: The Wireless Wired Equivalent Privacy (WEP) protocol is a stream cipher.
The simplest type of stream cipher is a substitution cipher. Substitution ciphers
simply substitute one letter or character for another as shown in below given figure.
Sometimes known as mono alphabetic substitution cipher, stream cipher can be
easy to break. A homo alphabetic substitution cipher maps a single plaintext
character to multiple cipher text characters

Fig: Transposition Cipher

Odisha State Open University Page 55

 With most symmetric ciphers, the final step is to combine the cipher stream with
the plaintext to create the ciphertext which is shown in figure below.

Fig: Combine Cipher

The process of accomplished through the exclusive OR (XOR) binary logic

operations because all encryption occurs in binary. XOR is used to combine two
streams of bits into one with a modified addition process. If two corresponding bits to
be added are the same, the result is 0; if the bits are different, the result is a 1. Instead
of combining the cipher stream with the plain text, a variation is to create a truly
random key (called a pad) to be combined with the plain text. This is known as a one-
time pad (OTP). If the pad is a random string of numbers that is kept secret and not
reused, then an OTP can be considered secure. OTPs are rarely used and are more
theoretical than practical.
3.4.2 Block Cipher
The second category of algorithms is known as a block cipher. Whereas a stream
cipher works on one character at a time, a block cipher manipulates an entire block of
plaintext at one time.
The plaintext message is divided into separate block of 8 to 16 bytes, and then each
block is encrypted independently. For additional security, the blocks can be
randomized. Stream and block ciphers each have advantages and disadvantages. A
stream cipher is fast when the plaintext is short, but can consume much more
processing power if plaintext itself. Because of this consistency, an attacker can
examine streams and may be able to determine the key. Block ciphers are considered

Page 56 Odisha State Open University

more secure because the output is more random. When using a block cipher, the
cipher is reset to its original state after each block is processed. This results in the
ciphertext being more difficult to break. Symmetric cryptography can provide
strong protection
against attacks as long the key is kept secure. The protections provided by
symmetric cryptography are summarized in below given table.
Table: Information protections by symmetric cryptography

3.4.3 Data Encryption System (DES)

One of the first widely popular symmetric cryptography algorithms was the Data
Encryption system (DES). The predecessor of DES was a product originally
designed in the early 1970s by
IBM called Lucifer that had a key length of 128 bits. The key was later shortened to
56 bits and renamed DES. The U.S. government officially adopted DES as the
standard for encrypting non classified information. DES effectively catapulted the
study of cryptography into the public arena. Until the deployment of DES,
cryptography was studied almost exclusively by military personnel. The
popularity of DES helped move cryptography implementation and research to
academic and commercial organizations. DES is block cipher. It divides plaintext
into 64-bit blocks and then executes the algorithm 16 times. There are four modes
of DES encryption.
Although DES was once widely implemented, its 56-bit key is no longer
considered secure and has been broken several times. It is not recommended for
use.

Odisha State Open University Page 57

 3.4.4 Triple Data Encryption Standard (3DES)
Triple Data Encryption Standard (3DES) is designed to replace DES. As its name
implies. 3DES uses three of encryption instead of just one. The ciphertext of one
round becomes the entire input for the second iteration. 3DES employs a total of 48
iterations in its encryption (3 iterations times' 16 rounds). The most secure version of
3DES use different keys for each round, as shown in figure below. In some versions
of 3DES, only two keys are used, but the first key is repeated for the round of
encryption. The version of 3DES that uses three keys is estimated to be 2 to the power
of 56 times stronger than DES.

Fig: 3DES

Although 3DES address several of the key weakness of DES, it is no longer

considered the most secure symmetric cryptographic algorithm. By design,
3DES performs better in hardware than as software.

Page 58 Odisha State Open University

3.4.5 Advance Encryption Standard (AES)
The Advance Encryption Standard (AES) is a symmetric cipher that was approved
by the NIST in late 2000 as a replacement for DES. The process began with the
NIST publishing requirements for a new symmetric algorithm and requesting
proposals. After a lengthy process that required the cooperation of the U.S.
Government, industry, and higher education, five finalists were chosen, with the
ultimate winner being an algorithm known as AES. AES is now the official
standard for encryption by the U.S. Government.
AES performs three steps on every block (128 bits) of plaintext. Within Step-2,
multiple rounds are performed depending on the key size: a 128-bit key performs 9
rounds; a192-bit key performs 11 rounds, and a 256-bit key, known as AES-256,
uses 13 rounds. Within each round, bytes are substituted and rearranged, and then
special multiplication is performed based on the new arrangement. AES is designed
to be secure well into the future. To date, no attacks have been successful against
AES.
3.4.6 Other Algorithms
Several other symmetric cryptographic algorithms are also used. Rivest Cipher
(RC) is a family of cipher algorithms designed by Ron Rivest. He developed six
ciphers, ranging from RC1 to
Rc6 (but did not release RC1 and RC3). RC2 is block cipher that process a block of
64 bits. RC4 is a stream cipher that accepts keys up to 128 on wireless LANs. RC5
is a block cipher that can accept blocks and keys of different lengths. RC6 has three
key sizes 128, 192, and 256 bits) and performs 20 rounds on each block. The
International Data Encryption Algorithm (IDEA) algorithm dates back to the early
1990s and used in European nations. It is a block cipher that processes 64 bits with a
128-bite key with 8 rounds. Although considered to be secure, a weak key of all
zeros has been identified for this algorithm. The algorithm Blowfish is block cipher
that operates on 64-bit block and can have a key length from 32 to 448 bits.
Blowfish was designed to run efficiently on 32-bit computers. To date, no
significant weaknesses have been identified. A later derivation of Blowfish known
as Twofishis also considered being a strong algorithm, although it has not been used
as widely as Blowfish.

Odisha State Open University Page 59

 3.5 ASYMMETRIC CRYPTOGRAPHIC ALGORITHMS

If Bob wants to send an encrypted message to Alice using symmetric encryption, he

must be sure that she has the key to decrypt the message. Yet how should Bob get the
key to Alice? He cannot send it electronically through the Internet, because that
would make it vulnerable to be intercepted by attackers. Nor can he encrypt the key
and send it. Because Alice would not have a way to decrypt the encrypted key. These
illustrate the primary weakness of symmetric encryption algorithms; distributing
and maintaining a secure single key among multiple users often scattered
geographically poses significant challenges. A completely different approach from
symmetric cryptography is asymmetric cryptographic algorithms, also known as
public key cryptography. Asymmetric encryption uses two keys instead of only one.
These keys are mathematically related and are known as the public key and the
private key. The public key is known to everyone and can be freely distributed, while
the private key is known only to the individual to whom it belongs. When Bob wants
to send a secure message to Alice, he uses Alice' public key to encrypt the message.
Alice then uses her private key to decrypt it. Asymmetric cryptography is illustrated
in figure below.

Fig: Asymmetric (Public key) cryptography

Page 60 Odisha State Open University

Asymmetric encryption was developed by Whitfield Diffie and Martin Hellman of
the Massachusetts Institute of Technology (MIT) in 1975.
There are several important principles regarding asymmetric cryptography:
Key Pairs: Unlike symmetric cryptography that uses only one key, asymmetric
cryptography requires a pair of keys.
Public Key: Public keys by their nature are designed to be ―public and do not
need to be protected. They can be freely given to anyone or even posted on the
Internet.
Private Key: The Private Key should be kept confidential and never shared. Both
directions. Asymmetric cryptography keys can work in both directions.
A document encrypted with a public key can be decrypted with the corresponding
private key.
In the same way, a document encrypted with a private key can be decrypted with its
public key. Asymmetric cryptography can also be used to provide proofs, suppose
that Alice receives an encrypted document that says it came from Bob. Alice can be
sure that the encrypted message was not viewed or altered by someone else while
being transmitted, yet how can she know for certain that Bob was actually the
sender? Because Alice's public key is widely available, anyone could use it to
encrypt the document. Another individual could have created a fictitious
document, encrypted it with Alice's public key, and then sent it to Alice while pre
tending to be Bob. While Alice's key can verify that no one read or changed the
document in transport, it cannot verify the sender.
Proof can be provided with asymmetric cryptography by creating a digital
signature, which is an electronic verification of the sender. A handwritten signature
on paper document servers as proof that the signer has read and agreed to the
document. A digital signature is much the same, although it can provide additional
benefits.
A digital signature can do the following:
· Verify the sender. A digital signature serves to confirm the identity of the person
from whom the electronic message originated.
· Prevent the sender from disowning the message. The signer cannot later
attempt to disown it by claiming the signature was forged.
· Prove the integrity of the message. A digital signature can also prove that the
message has not been altered since it was signed.
The basis for a digital signature rests on the ability of asymmetric keys to work in

Odisha State Open University Page 61

 both directions (a public key can encrypt a document that can be decrypted with a
private key, and the Private Key can encrypt a document that can be decrypted by the
public key). The steps for Bob to send a digitally signed message to Alice are
illustrated in figure below.

Fig: Digital Signature

1. After creating a memo, Bob generates a hash on it.

2. Bob then encrypts the hash with his private key. This encrypted hash is the
digital signature for the memo.
3. Bob sends both the memo and the digital signature to Alice.
4. When Alice receives them, she decrypts the digital signature using Bob's public
key, revealing the hash. If she cannot decrypt the digital signature, then she knows
that it did not come from Bob (because only Bob's public key is able to decrypt the
hash generated with his private key).
5. Alice then hashes the memo with the same hash algorithm Bob used and
compares the result to the hash she received from Bob. If they are equal, Alice can be
confident that the message has not changed since he signed it. Yet if the hashes
6. are not equal, the message has changed since it was signed.

Page 62 Odisha State Open University

 Fig: Asymmetric Cryptography Practices

3.6 RSA ALGORITHM

The asymmetric algorithm RSA was published in 1977 and patented by MIT in
1983. The RSA Algorithm is the most common asymmetric cryptography
algorithm and is the basis for several products. RSA stands for last names of its
three developers, Ron Rivest, Adi Shamir and Leonard Adleman.
The RSA algorithm multiples two large prime numbers (a prime number is a
number divisible only by itself and 1), p and q, to compute their product (n=pq)).
Next a number e is chosen that is less than n and a prime factor to (p-1)(q-1).
Another number d is determined, so that (ed-1) is divisible by (p-1) (q-1). The
values of e and d are the public and private exponents. The public key is the pair
(n.e), while the private key is (n,d). The numbers p and q can be discarded. An
illustration of the RSA algorithm using very small number is as follows:

Odisha State Open University Page 63

 i. Select two prime numbers, p and q (in this example), p=7 and q=19).
ii. multiply p and q together to create n (7*19=133)
iii. Calculate m as p-1* q-1([7-1] * [19-1] or 6 * 18 = 108)
iv. Find a number e so that it and m have no common positive divisor other than
1(5)
v. Find a number d so that d=1+n*m)/e ([1+3*108]/5 or 325/5 = 65
For this example, the public key n is 133 and e is 5, while for the private key, n is133
and d is 65.
Note: RSA is slower than other algorithms; DES is approximately 100 times faster
than RSA in software and between 1,000 and 10,000 times as fast in hardware.
3.6.1 Elliptic curve Cryptography (ECC)
Elliptic curve Cryptography (ECC) was first proposed in the mid-1980s. Instead of
using large prime numbers as with RSA, elliptic curve cryptography uses sloping
curves. An elliptic curve is a function drawn on an X-Y axis as a gently curved line.
By adding the values of two points on the curve, a third point on the curve can be
derived. With ECC, users share one elliptic curve and one point on the curve. One
user chooses a secret random number and computes a public key based on a point on
the curve, the other user does the same. They can now exchange messages because
the shared public keys can generate a private key on an elliptic curve.
a) ECC is considered an alternative for prime numbers-based asymmetric
cryptography for mobile and wireless devices. Because mobile devices are limited
in terms of computing power due to their smaller size, ECC offers security that is
comparable to other asymmetric cryptography, but with smaller key sizes. This can
result in faster computations and lower power consumption.
b) Another asymmetric algorithm known as the Diffie-Heliman algorithm does
not encrypt and decrypt text. Rather, the strength of Diffie-Hellman is that it allows
two users to share a secret key securely over a public network. Once the key has been
shared, then both parties can use it to encrypt and decrypt messages using symmetric
cryptography.
3.7 USING CRYPTOGRAPHY

Cryptography should be used to secure any and all data that needs to be protected.
This includes individual files or databases that are stored on standard desktop
computer servers, removable media or mobile devices. Cryptography can be

Page 64 Odisha State Open University

applied through either software or hardware.
3.7.1 Encryption through Software
Encryption can be implemented through cryptographic software running on a
system. This can be applied to individual files by using the software to encrypt and
decrypt each file. The encryption can also be performed on a larger scale through
using the file system or by encrypting the entire disk drive.
3.7.2 File and File System Cryptography
Encryption software can be used to encrypt or decrypt files one by one. However,
this can be a cumbersome process. Instead, protecting groups of files, such as all
files in a specific folder, can take advantage of the operating system's file system.
A file system is a method used by operating system to store, retrieve and organize
files. Protecting individual files or multiple files through file system cryptography
can be performed using software such as pretty Good Privacy and Microsoft
Windows Encrypting File System.
3.7.3 Pretty Good Privacy (PGP/GPG)
One of the most widely used asymmetric cryptography systems for files and e-mail
messages on windows systems is a commercial product called Pretty Good
Privacy (PGP). A similar program known as GNU Privacy Guard (GPG) is an
open source product. GP windows, UNIX and Linux Operating systems.
Messages encrypted by PGP can generally be decrypted by GPG and vice versa. G
versions run on windows, UNIX and Linux Operating systems. Messages
encrypted by PGP can generally be decrypted by GPG and vice versa.
a) PGP and GPG use both asymmetric and symmetric cryptography.
PGP/GPG generates a random symmetric key and uses it to encrypt the message.
The symmetric key is then encrypted using the receiver's public key and sent along
with the message. When the recipient receives a message, PGP/GPG first decrypts
the symmetric key with the recipient's private key. The decrypted symmetric key is
then used to decrypt the rest of the message.
b) PGP uses symmetric cryptography because it is faster than asymmetric
cryptography.
c) PGP uses RSA for protecting digital signatures and 3DES or IDEA for
symmetric encryption. GPG is unable to use IDEA because IDEA is patented.
Instead, GPG uses one of several open-source algorithms.

Odisha State Open University Page 65

 3.8 SELFASSESSMENT QUESTIONS

1. What are the principle elements of a public key cryptosystem?

………………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
2. What is a hash function?
………………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………

3. What are the requirements of the hash function?

………………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
4. What are the properties a digital signature should have?
………………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………
5. What requirements should a digital signature scheme should satisfy?
………………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
6. Discuss about Data Encryption Standard (DES)
………………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………

Page 66 Odisha State Open University

7. Explain RSA with the help of an example?
………………………………………………………………………..................
………………………………………………………………………..................
………………………………………………………………………..................
………………………………………………………………………..................
………………………………………………………………………..................

3.9 KEY TERMS & CONCEPTS

A Cryptographic Hash Function takes a message of arbitrary length and

creates a message digest of fixed length.
Message Digest (MD) is a series of byte-oriented algorithms that produce a 128-
bit hash value from an arbitrary-length message.
Secure Hash algorithm (SHA) is a family of hashes. It is a more secured hash
than MD.
Whirlpool is an iterated cryptographic hash function, based on the Miyaguchi-
Preneel scheme that uses a symmetric-key block cipher in place of the
compression function.
Data Encryption Standard (DES) is a block cipher. It divides plaintext into 64-
bit blocks and then executes the algorithm 16 times for encryption.
Advance Encryption Standard (AES) is a symmetric cipher that was approved
by the NIST. AES is now the official standard for encryption by the U.S.
Government. AES performs three steps on every block (128 bits) of plaintext at a
time.
RSA stands for last names of its three developers, Ron Rivest, Adi Shamir and
Leonard Adelman, was published in 1977 and patented by MIT in 1983. The RSA
Algorithm is the most common asymmetric cryptography algorithm.

3.10 ANSWER TO SELFASSESSMENT QUESTIONS

1. The principle elements of a cryptosystem are:

(i) Plain text
(ii) Encryption algorithm

Odisha State Open University Page 67

 (iii) Public and private key
(iv) Cipher text
(v) Decryption algorithm
2. Hash function accept a variable size message M as input and produces a fixed
size hash code H(M) called as message digest as output. It is the variation on the
message authentication code.
3. The hash function should satisfy the following requirements.
(i) H can be applied to a block of data of any size.
(ii) H produces a fixed length output.
(iii) H(x) is relatively easy to compute for any given x, making both hardware and
software implementations practical
4. The properties of digital signature include the following:
(i) It must verify the author and the data and time of signature.
(ii) It must authenticate the contents at the time of signature.
(iii) It must be verifiable by third parties to resolve disputes.
5. The requirements of a digital signature include the following:
(i) The signature must be bit pattern that depends on the message being signed.
(ii) The signature must use some information unique to the sender, to prevent
both forgery and denial.
(iii) It must be relatively easy to produce the digital signature.
(iv) It must be relatively easy to recognize and verify the digital signature.
(v) It must be computationally infeasible to forge a digital signature, either by
constructing a new message for an existing digital signature or by constructing a
fraudulent digital signature for a given message.
(vi) It must be practical to retain a copy of the digital signature in storage.
6. The Data Encryption Standard (DES) is a symmetric-key block cipher
published by the National Institute of Standards and Technology (NIST) in 1977.
DES has been the most widely used symmetric-key block cipher since its
publication.
DES takes 64 bit plain text converts it into 64 bit cipher text with the help of 64 bit
key which later reduced to 56 bit key as every 8th bit of 64 bit is discarded to form a
key of 54 bit. As DES is a block cipher, it takes plain text as block of 64 bit.
7. RSA is an asymmetric block cipher. It was developed by Ron Rivest, Adiv
Shamir and Leonard Adleman in 1977.
RSA with an example:

Page 68 Odisha State Open University

Let us choose two large prime numbers p=61 and q= 53
· Multiply p and q together to get n =61*53=3233
· Choose the encryption key e, such that e and (p - 1) x (q - 1) are relatively
prime.
· (p-1) =(61-1)=60
· (q-1) =(53-1)=52
· (p - 1) × (q - 1) =60*52=3120
· Choosing a relatively prime number between 1<e<3120 which is not a
multiple of 3120. We can choose e=17
· Compute decryption key d such that d = 17 mod (3120) = 2753
· Construct public key as (17, 3233) and construct cipher text,
c = 6517 mod (3233) =2790
· Construct private key as (2753, 3233) and construct plain text,
p =27902753 mod (3233) =65

3.11 REFERENCES AND FURTHER READINGS

1. William Stallings, “Cryptography and Network Security-Principles and

Practices”, Prentice-Hall of India.
2. Information System Security (CEGCS-04), Study Materials of Uttarakhand
Open University, Haldwani, for Certificate in e e-Governance and Cyber Security.
3. Charles P.Pfleeger, Shari Lawrence Pfleeger, even Shah, “Security in
Computing”, Pearson Education
4. Bernad Menezes, “Network Security & Cryptography”, CENGAGE
Learning.
5. Atul Kahate, “Cryptography and Network Security”, TMH Publishing
Company Limited.

Odisha State Open University Page 69

 Comments

Page 70 Odisha State Open University