7/5/21

Cryptography and Network Security Basic Concepts JNTU K UNIT-1

 Cryptography and Network Security Basic Concepts

INTRODUCTION

Computer data often travels from one computer to another, leaving the safety of its protected physical surroundings. Once the data is out of hand, people with bad intention could modify or forge your data, either for amusement or for their own benefit. Cryptography can reformat and transform our data, making it safer on its trip between computers. The technology is based on the essentials of secret codes, augmented by modern mathematics that protects our data in powerful ways.

• Computer Security - generic name for the collection of tools designed to protect data and to thwart hackers

• Network Security - measures to protect data during their transmission

• Internet Security - measures to protect data during their transmission over a collection of interconnected networks

Basic Concepts

Cryptography The art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back to its original form

Plaintext The original intelligible message

Cipher text 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. Also called code breaking

Cryptology Both cryptography and cryptanalysis

Code An algorithm for transforming an intelligible message into an unintelligible one using a code-book

Cryptography

Cryptographic systems are generally classified along 3 independent dimensions:

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 whichelements in the plaintext are rearranged.

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.

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.

Basic Principles Security Goals:



The primary goal of network security are Confidentiality, Integrity, and Availability. These three pillars of Network Security are often represented as CIA triangle.

·        Confidentiality − The function of confidentiality is to protect precious business data from unauthorized persons. Confidentiality part of network security makes sure that the data is available only to the intended and authorized persons.

·        Integrity − This goal means maintaining and assuring the accuracy and consistency of data. The function of integrity is to make sure that the data is reliable and is not changed by unauthorized persons.

·        Availability − The function of availability in Network Security is to make sure that the data, network resources/services are continuously available to the legitimate users, whenever they require it.

 Cryptographic Attacks:


 


Passive Attacks

These attacks are not very dangerous as they do not cause any modification to the data. These attacks are generally done to secretly listen and monitor the communication of other parties. Passive attacks are very difficult to detect because these attacks do not change the information of the data.
There are two types of passive attacks in cryptography and network security: Traffic analysis and Release of Message content

  • Traffic Analysis: In this attack, an attacker tries to predict the nature of communication by using information. The information such as analyzing traffic, identify communication hosts, and frequency of messages.

 

  • Release of Message content: It is similar to hearing a telephone conversation between two users. In this attack, the attacker can monitor the content of the transmitted data such as email messages, etc.

  



Active attacks

These attacks involve some modification of the data stream or the creation of a false stream. These attacks can be classified in to four categories:

Masquerade – One entity pretends to be a different entity.

Replay – involves passive capture of a data unit and its subsequent transmission to produce an

unauthorized effect.

Modification of messages – Some portion of message is altered or the messages are delayed or

recorded, to produce an unauthorized effect.

Denial of service – Prevents or inhibits the normal use or management of communication facilities. Another form of service denial is the disruption of an entire network, either by disabling the network or overloading it with messages so as to degrade performance. It is quite difficult to prevent active attacks absolutely, because to do so would require physical protection of all communication facilities and paths at all times. Instead, the goal is to detect them and to recover from any disruption or delays caused by them.





SECURITY ATTACKS

There are four general categories of attack which are listed below.

Interruption

An asset of the system is destroyed or becomes unavailable or unusable. This is an attack on availability e.g., destruction of piece of hardware, cutting of a communication line or Disabling of file management system.

Interception

An unauthorized party gains access to an asset. This is an attack on confidentiality. Unauthorized party could be a person, a program or a computer. e.g., wire tapping to capture data in the network, illicit copying of files.

Modification

An unauthorized party not only gains access to but tampers with an asset. This is an attack on integrity. e.g., changing values in data file, altering a program, modifying the contents of messages being transmitted in a network.

Fabrication

An unauthorized party inserts counterfeit objects into the system. This is an attack on authenticity. e.g., insertion of spurious message in a network or addition of records to a file.

 

Services and Mechanisms:

      Confidentiality

      Authentication

      Integrity

      Non-repudiation

      Access control

      availability

  


Note: Interception causes  loss of message  confidentiality

Authentication: Authentication is the process of recognizing a user’s identity. It is the mechanism of associating an incoming request with a set of identifying credentials.


Note: Fabrication is possible in absence of authentication

Integrity:
1.Prevent unauthorized users from making modifications to data or programs.
2. Prevent authorized users from making improper or unauthorized modifications.
3.Maintain internal and external consistency of data and  programs.


Note: Modification causes loss of message integrity

Non repudiation:

Non repudiation does not allow the sender of a message to refute the claim of not sending that message.

 


 Access control

      Access control specifies and controls who can access what

      An access control mechanism controls which clients or applications have access to the server. Only properly authorized clients can connect to the server.

      Two different types of access control mechanisms are used: user based and host based. 

Availability : means that information is accessible to authorized users. It is basically an assurance that your system and data are accessible by authorized users whenever it's needed.



Security mechanisms:




Symmetric and public key algorithms

Encryption/Decryption methods fall into two categories.

Symmetric key

Public key

In symmetric key algorithms, 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.

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.

A MODEL FOR NETWORK SECURITY

A message is to be transferred from one party to another across some sort of internet. The two

parties, who are the principals in this transaction, must cooperate for the exchange to take place.

A logical information channel is established by defining a route through the internet from source

to destination and by the cooperative use of communication protocols (e.g., TCP/IP) by the

two principals.

CONVENTIONAL ENCRYPTION

• Referred conventional / private-key / single-key

• Sender and recipient share a common key


Here the original message, referred to as plaintext, is converted into apparently random

nonsense, referred to as cipher text. The encryption process consists of an algorithm and a key.

The key is a value independent of the plaintext. Changing the key changes the output of the

algorithm. Once the cipher text is produced, it may be transmitted. Upon reception, the

cipher text can be transformed back to the original plaintext by using a decryption algorithm

and the same key that was used for encryption. The security depends on several factors. First, the

encryption algorithm must be powerful enough that it is impractical to decrypt a message on

the basis of cipher text alone. Beyond that, the security depends on the secrecy of the key,

not the secrecy of the algorithm.

• Two requirements for secure use of symmetric encryption:

– A strong encryption algorithm

– A secret key known only to sender / receiver

Y = EK(X)

X = DK(Y)

• assume encryption algorithm is known

implies a secure channel to distribute key

CLASSICAL ENCRYPTION TECHNIQUES

There are two basic building blocks of all encryption techniques: substitution and

transposition.

SUBSTITUTION TECHNIQUES

A substitution technique is one in which the letters of plaintext are replaced by other letters or by

numbers or symbols. If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with cipher text bit patterns.

Caesar cipher (or) shift cipher

The earliest known use of a substitution cipher and the simplest was by Julius Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter standing 3 places

further down the alphabet.

e.g., plain text : pay more money

Cipher text: SDB PRUH PRQHB

Note that the alphabet is wrapped around, so that letter following „z‟ is „a‟.

For each plaintext letter p, substitute the cipher text letter c such that

C = E(p) = (p+3) mod 26

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

C = E (p) = (p+k) mod 26

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

P = D(C) = (C-k) mod 26

Playfair 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 cipher text digrams. The playfair

algorithm is based on the use of 5x5 matrix of letters constructed using a keyword. Let the

keyword be „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 ofthe matrix with the remaining letters in alphabetical order.The letter „i‟ and „j‟ count as one letter. Plaintext is encrypted two letters at a time

According to the following rules:

Repeating plaintext letters that would fall in the same pair are separated with a

Filler letter such as „x‟.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 following the last.

Plaintext letters that fall in the same column are replaced by the letter beneath, with the top

element of the column following the last.

Otherwise, each plaintext letter is replaced by the letter that lies in its own row

And the column occupied by the other plaintext letter.

 


Plaintext = meet me at the school house

Splitting two letters as a unit => me et me at th es ch o x ol ho us ex

Corresponding cipher text => CL KL CL RS PD IL HY AV MP HF XL IU

Strength of playfair cipher

Playfair cipher is a great advance over simple mono alphabetic ciphers.

Since there are 26 letters, 26x26 = 676 diagrams are possible, so identification of individual

diagram is more difficult.

1.15.1.3 Polyalphabetic ciphers

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

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. The process of

 


 

 

Encryption is simple: Given a key letter X and a plaintext letter y, the cipher text is at theintersection of the row labeled x and the column labeled y; in this case, the ciphertext is V.

To encrypt a message, a key is needed that is as long as the message. Usually, the key is a repeating keyword.

e.g., 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 av 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.

Strength of Vigenere cipher

o There are multiple cipher text letters for each plaintext letter.

o Letter frequency information is obscured.

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 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 Ci - ith binary digit of cipher text Pi - ith binary digit of

plaintext Ki - ith 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.

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

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:

m e a t e c o l o s

e t t h s h o h u e

The encrypted message is

MEATECOLOSETTHSHOHUE

Row Transposition Ciphers-

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

PT = m e e t a t t

h e s c h o o

l h o u s e

CT = ESOTCUEEHMHLAHSTOETO

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.

Feistel cipher structure

The input to the encryption algorithm are a plaintext block of length 2w bits and a key K.

the plaintext block is divided into two halves L0 and R0. The two halves of the data pass through „n‟ rounds of processing and then combine to produce the ciphertext block. Each round „i‟ has inputs Li-1 and Ri-1, derived from the previous round, as well as the subkey Ki, derived from the overall key K. in general, the subkeys Ki are different from K and from each other.

All rounds have the same structure. A substitution is performed on the left half of the data (as similar to S-DES). This is done by applying a round function F to the right half of the data and then taking the XOR of the output of that function and the left half of the data. The round function has the same general structure for each round but is parameterized by the round sub key ki. Following this substitution, a permutation is performed that consists of the interchange of the

two halves of the data. This structure is a particular form of the substitution-permutation network.

The exact realization of a Feistel network depends on the choice of the following parameters and

design features:

Block size - Increasing size improves security, but slows cipher

Key size - Increasing size improves security, makes exhaustive key searching harder, but may

slow cipher

Number of rounds - Increasing number improves security, but slows cipher

Subkey generation - Greater complexity can make analysis harder, but slows cipher

Round function - Greater complexity can make analysis harder, but slows cipher

Fast software en/decryption & ease of analysis - are more recent concerns for practical use and testing.

Fig: Classical Feistel Network


Fig: Feistel encryption and decryption

The process of decryption is essentially the same as the encryption process. The rule is as follows:

use the cipher text as input to the algorithm, but use the subkey ki in reverse order. i.e., kn in the first round, kn-1 in second round and so on. For clarity, we use the notation LEi and REi for data traveling through the decryption algorithm. The diagram below indicates that, at each round, the intermediate value of the decryption process is same (equal) to the corresponding value of the encryption process with two halves of the value swapped.

i.e., REi || LEi (or) equivalently RD16-i || LD16-i

After the last iteration of the encryption process, the two halves of the output are

swapped, so that the cipher text is RE16 || LE16. The output of that round is the cipher text. Now take the cipher text and use it as input to the same algorithm. The input to the first round is RE16 || LE16, which is equal to the 32-bit swap of the output of the sixteenth round of the encryption process.

Now we will see how the output of the first round of the decryption process is equal to a32-bit swap of the input to the sixteenth round of the encryption process. First consider the encryption process,

LE16 = RE15

RE16 = LE15 F (RE15, K16) On the decryption side,

LD1 =RD0 = LE16 =RE15

RD1 = LD0 F (RD0, K16)

= RE16 F (RE15, K16)

= [LE15 F (RE15, K16)] F (RE15, K16)

= LE15

Therefore, LD1 = RE15

RD1 = LE15 In general, for the ith iteration of the encryption algorithm, LEi = REi-1

REi = LEi-1 F (REi-1, Ki)

Finally, the output of the l

BLOCK CIPHER PRINCIPLES

Virtually, all symmetric block encryption algorithms in current use are based on a structure

referred to as Fiestel block cipher. For that reason, it is important to examine the design principles

of the Fiestel cipher. We begin with a comparison of stream cipher with block cipher.

• A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. E.g,

vigenere cipher. A block cipher is one in which a block of plaintext is treated as a whole and

used to produce a cipher text block of equal length. Typically a block size of 64 or 128 bits is

used.

Block cipher principles

• most symmetric block ciphers are based on a Feistel Cipher Structure needed since must be

able to decrypt ciphertext to recover messages efficiently. block ciphers look like an extremely

large substitution

• would need table of 264 entries for a 64-bit block

• Instead create from smaller building blocks

• using idea of a product cipher in 1949 Claude Shannon introduced idea of substitution-

permutation (S-P) networks called modern substitution-transposition product cipher

these form the basis of modern block ciphers

• S-P networks are based on the two primitive cryptographic operations we have seen before:

substitution (S-box)

permutation (P-box)

• provide confusion and diffusion of message

• diffusion – dissipates statistical structure of plaintext over bulk of ciphertext

confusion – makes relationship between ciphertext and key as complex as possible

A=0,B=1,C=2,D=3,E=4,F=5,G=6,H=7,I=8,J=9,K=10,L=11,M=12,N=13,O=14,P=15,Q=16,R=17,S=18,T=19,U=20,V=21,W=22,X=23,Y=24,Z=25

A)What is(A+N) MOD26 In this System0+13=13=N

B)What is(C-10) MOD26 in this System

2-10=-8+26=18=S

C)What is(W+Q) MOD26 in this System

= 22+16=38%26=12=M

 

Find the particular and the general solutions to the following linear Diophantine equation 19x +13y = 20.

 

Gcd(19,13)=1

ax + by = c.

Particular solution: x0 = (c/d)s and     y0 = (c/d)t

General solutions: x = x0 + k (b/d) and  y = y0 − k(a/d)

34/22=>34=1*22+12

R1=q*R2+r

s=s1-qs2                          t=t1-qt2

R1

R2

r

S1

S2

s

T1

T2

t

q

19

13

6

1

0

1

0

1

-1

1

13

6

1

0

1

0-2*1=-2

1

-1

1-2(-1)=3

2

6

1

0

1

-2

1-6*-2=13

-1

3

-1-6*3=-19

6

1

0

 

-2

13

 

3

-19

 

 

Gcd(19,13)=1

 

S=-2

T=3

Particular solution: x0 = (c/d)s and     y0 = (c/d)t

General solutions: x = x0 + k (b/d) and  y = y0 − k(a/d)

 

X0=(20/1)* -2=-40

Y0=(20/1)*3=60

X0=-40

 Y0=60

X=-40+13K

Y=60-19K

Find the particular and the general solutions to the following linear Diophantine equation 25x +10y = 15.

 

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