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118 Pages·2006·2.11 MB·English

by  Anuj Prateek

A REPORT

ON

BLOCK
CIPHERS

(Literature Survey)
A Report on
BLOCK CIPHERS
(Literature Survey)

ANUJ PRATEEK R GURUPRASAD

B ITS, PILANI NAL, BANGALORE

Reviewed by:
Dr Vidyadhar Mudkavi
NAL, BANGALORE

OCTOBER 2006

N A L
ATIONAL EROSPACE ABORATORIES
BANGALORE

ACKNOWLEDGEMENT

ACKNOWLEDGEMENT

I would like to extend my deep gratitude to Dr. A. R. Upadhya, Director, NAL and
Dr. Ranjan Moodithaya, Head KTMD, NAL for granting me the permission and
resources, which were indispensable for writing this report.

I am also extremely grateful to Dr. M. R. Nayak, Head, TS and Dr. R. M. Jha,
Scientist, ALD for their kind permission and very useful suggestions to work in this
very interesting area.

I would also like to express my deep gratitude to Dr. Vidyadhar Mudkavi, Scientist,
CTFD and Dr. U. N. Sinha, Head, FSD for their guidance, technical help and constant
motivation throughout the writing of the report.

I would like to extend my acknowledgement to Dr. S. Bhogle who provided the initial
motivation to carry out this report.

Last but not the least; I would like to extend my special thanks to Mr. R. Guruprasad
for contributing immensely to the SECTION ONE. It would not be wrong to say that
it is completely his work. I would also like to thank him for providing me various
resources from his lab and in person too.
CONTENT

CONTENT

S.No. TITLE Page No.

A ABSTRACT & INTRODUCTION I

B SECTION ONE III
1 INTRODUCTION TO CRYPTOGRAPHY 1
2 EVOLUTION OF CRYPTOGRAPHY 6
3 CHARACTERSTICS OF A GOOD CIPHER 9
REFERENCES R-I

C SECTION TWO IV
4 PRODUCT CIPHERS 11
5 BLOCK MODES 13
6 FEISTEL NETWORK 17
7 S-BOX 19
REFERENCES R-II

D SECTION THREE V
8 LUCIFER 23
9 DES 25
10 IDEA 32
11 CAST 34
12 LOKI 37
13 SERPENT 41
14 DEAL 45
15 MARS 47
16 SQUARE 55
17 AES 58
18 ANUBIS 66
19 CMEA 72
20 PHELIX 74
21 TIGER 78
22 OTHER ALGORITHMS 80
REFERENCES R-III

E SECTION FOUR VI
23 CRYPTANALYSIS 84
24 DIFFERENTIAL CRYPTANALYSIS 96
25 LINEAR CRYPTANALYSIS 99
REFERENCES R-V

26 CONCLUSION 100
27 FUTURE APPLICATION 100
ABSTRACT & INTRODUCTION

ABSTRACT

This document is a literature survey about block ciphers, especially based on Feistel
Network, presented in form of a report. The report concentrates on providing a
starting point for designing strong, secure, and efficient cryptosystems. Various
design issues and algorithms have been described in the report. Description about
various forms of cryptanalysis has also been provided. Report explains the key
players of design of block ciphers in detail. The report will provide as a excellent
reference material for anyone who wants to design a Feistel network based block
cipher.

KEYWORDS: Evolution, Good Cipher Characteristics, Product Ciphers, Feistel
Network, S-Box, LUCIFER, DES, IDEA, CAST, LOKI97, SERPENT, DEAL, MARS,
SQUARE, AES, ANUBIS, CMEA, PHELIX, TIGER, Cryptanalysis, Differential &
Linear Cryptanalysis

INTRODUCTION

Horst Feistel in his famous article, “Cryptography & Computer Privacy,” published in
Scientific American, in May 1973, rightly wrote in the very first paragraph of the
article, “There is growing concern that computers now constitute a dangerous threat to
individual privacy. Since many computers contain personal data and are accessible
from distant terminals, they are viewed as unexcelled means of assembling large
amount of information about individual or a group. It is asserted that it will soon be
feasible to compile dossiers in depth of an entire citizenry, where until recently the
material for such dossiers was scattered in many separate locations under widely
diverse jurisdictions. It will be argued here, however, a computer system can be
adapted to guard its content from everyone but authorized individuals by enciphering
the materials in forms highly resistant to cipher breaking.”

Feistel clearly reflected in his words that as the amount of digital content is growing,
the need of secrecy and authorized access to the information is increasing and future
systems should be able to answer these needs. In modern era, these needs have been
answered by cryptography, the science and cryptosystem, the product. In these all
years the need has grown manifold in various forms and so has the elicit activities
related to acquiring unauthorized information has grown. Every science has its
advantages and disadvantages, so has cryptography. People involved in various illegal
businesses have started using cryptosystems to mask their activities and this has
forced law implementers to force the evolution of cryptanalysis i.e. the science of
unmasking the crypts. Not arguing on the pros and cons of cryptography, the thing
that remains as a concern for learners of this science is development of secure
cryptosystems.

Cryptography is divided into category and two of the broad categories of general
interest are private and public key cryptography. This report talks about private key
cryptography and in particular, the sub-sub domain known as Feistel Network based
block ciphers.

I
ABSTRACT & INTRODUCTION

The report has been divided into four parts, named as sections. The report flows in a
way that the key concepts that are required in design of mentioned cryptosystems are
touched and they appear in a progressive sequence. The report neither goes into deep
mathematical aspects like efficiency analysis and attack testing etc nor does it sways
away from the basic mathematics.

The first section introduces cryptography, evolution of cryptography and the desired
characteristics of a cryptosystem, in brief.

The second section briefs product ciphers, Modes of operation of block ciphers,
Feistel Network and S-boxes. Feistel Networks fall in the broad category of product
ciphers which is a sub-domain of block ciphers. As any block ciphers have various
modes of operation, they are introduced too in this section. S-boxes constitute a very
important part of Feistel Network, are most prone to attacks, and hence make place in
the section.

The third section, the major part of this report, presents various cryptographic
algorithms that have come in the modern age. Algorithms are introduced, explained
and the attacks have been mentioned. Algorithms that were picked for presentation
includes LUCIFER, DES, IDEA, CAST, LOKI, SERPENT, DEAL, MARS,
SQUARE, AES, ANUBIS, CMEA, PHELIX, and TIGER. These algorithms provide a
strong base for development of new algorithms and show the variety of attacks that
can be made on them. CMEA, PHELIX, and TIGER are odd among the mentioned
algorithms but have been included for completeness and the lessons that they teach.
CMEA provides an insight of embedded-efficient algorithms. PHELIX and TIGER
show the other side of private key encryption namely stream ciphers and use hashes.
At the end of this section, an introduction about various algorithms existing today is
mentioned, though the details are omitted as they are covered more or less by the
mentioned detailed algorithms. The last part of the section will help in improving the
general knowledge about the algorithms, will provide with the names of key people in
the field, and lastly will provide a place to look for reference.

The last section or section four introduces cryptanalysis. The section briefs about
various methodologies of cryptanalysis. In addition, at the end talks about differential
and linear cryptanalysis, the most powerful types of attacks on present day
cryptosystems.

The report will serve as an excellent reference to anyone who wants to develop a
cryptosystem of the type mentioned, whether an advanced reader or an amateur. It is
not as everything related to the design has been covered but yes most have been and
after reading the report, the reader will know how to proceed and where to look for
help. Lastly, the report does not guarantee that the reader will be able to actualize the
concept, but yes, a step would be taken in direction of actualization.

“Every science needs effort but cryptography is not a mere science, but broader, it is
an art; one needs to feel it, imagine it, and breathe it.”

II
SECTION ONE

SECTION - I

Introduction to Cryptography
Evolution of Cryptography
Characteristics of a Good Cipher

- R. Guruprasad
- Anuj Prateek

III
INTRODUCTION TO CRYPTOGRAPHY

1. INTRODUCTION TO CRYPTOGRAPHY

The word “cryptography” is a Greek word and means “secret writing.” Earlier
cryptography was used primarily by the military for the purposes of espionage.
Cryptography is defined as the science of devising methods that allow information to
be sent in a secure form in such a way that the only person able to retrieve this
information is the intended recipient.

With the advances in modern communication, technology has enabled businesses and
individuals to transport information at a very low cost via public networks such as the
Internet. This development comes at the cost of potentially exposing the data
transmitted over such a medium. Therefore, it becomes imperative for businesses to
make sure that sensitive data is transferred from one point to another in an airtight,
secure manner over public networks. Cryptography helps to achieve this goal by
making messages unintelligible to all but the intended recipient.

Cryptography as a technique can be summarized by the set {P, C, K, E, D} where,

• P = Plaintext space
• C = Ciphertext Space
• K = Key Space
• E = Encryption Function Space
• D = De-Encryption Function Space

The basic principle is this: A message being sent is known as plaintext. The message
is then coded using a cryptographic algorithm. This process is called encryption. An
encrypted message is known as ciphertext, and is turned back into plaintext by the
process of decryption.

Figure 1.1 Schematic of cryptosystem
It must be assumed that any eavesdropper has access to all communications between
the sender and the recipient. A method of encryption is only secure if even with this
complete access, the eavesdropper is still unable to recover the original plaintext from
the ciphertext. In the last few decades, cryptographic algorithms, being mathematical
by nature, have become sufficiently advanced that computers can only handle them.
This in effect means that plaintext is binary in form, and can therefore be anything; a
picture, a voice, an e-mail or even a video. The actual mathematical function used to
encrypt and decrypt messages is called a cryptographic algorithm or cipher. This is
only part of the system used to send and receive secure messages.

1
INTRODUCTION TO CRYPTOGRAPHY

1.1 GROUPS OF ALGORITHMS
Cryptographic algorithms are classified into various groups and a brief description is
given here.
1.1.1 RESTRICTED ALGORITHM
If, as with most historical ciphers, the security of the message being sent relies on the
algorithm itself remaining secret, then that algorithm is known as a restricted
algorithm. One of the major drawbacks of such algorithm is that a large or changing
group of users cannot utilize them, as every time one user leaves the group, everyone
must change algorithm and if the algorithm is compromised in any way, a new
algorithm must be implemented.

1.1.2 KEY-BASED ALGORITHM

Practically all modern cryptographic systems make use of a key. Algorithms that use a
key system allow all details of the algorithm to be widely available. This is because
all of the security lies in the key. With a key-based algorithm, the plaintext is
encrypted and decrypted by the algorithm, which uses a certain key, and the resulting
ciphertext is dependant on the key, and not the algorithm.

1.1.3 SYMMETRIC ALGORITHM
Symmetric algorithms have one key that is used to both encrypt and decrypt the
message. This presents one major problem that is the transfer of the key between
encrypter and the decrypter can be compromised by the attacker. There are two types
of symmetric algorithms namely Stream and Block ciphers. Stream ciphers operate on
plaintext one bit at a time. Block ciphers operate on groups of bits called blocks. The
major advantages and disadvantages of this kind of algorithm are given next.
Advantages,
• Very fast relative to public key cryptography
• Considered secure, provided the key is relatively strong
• The ciphertext is compact

Disadvantages,

• The administration of the keys can become extremely complicated
• A large number of keys is needed to communicate securely with a large group
of people
• Non-repudiation is not possible
• The key is subject to interception by hackers
1.1.4 ASYMMETRIC ALGORITHM
Asymmetric algorithm is the algorithm in which the encryption and decryption keys
are different. The encryption key is known as the public key and the decryption key is

2