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A MODEL FOR NETWORK SECURITY

Two parties (principals) must work together for a successful information exchange.

A communication path is established using Internet routing and protocols like TCP/IP.

Security becomes important to protect data from threats such as breaches of confidentiality or authenticity.

All the techniques for providing security have two components:

A security-related transformation
Secret information

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There are four basic tasks in designing a particular security service:

Design an algorithm
Generate the secret information
Develop methods for the distribution and sharing of the secret information.
Specify a protocol

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Symmetric Cipher Model

A symmetric encryption scheme has five ingredients :

Plaintext
Encryption algorithm
Secret key
Cipher text
Decryption algorithm

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Cryptographic systems are characterized along three independent dimensions:

The type of operations used for transforming plaintext to ciphertext.

1.Substitution 2. Transposition

The number of keys used

1. Symmetric 2.Asymmetric

The way in which the plaintext is processed.

1. Block cipher 2. Stream cipher

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Cryptanalysis and Brute-Force Attack

Cryptanalysis: Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the plaintext or even some sample plaintext–ciphertext pairs.

Brute-force attack: The attacker tries every possible key on a piece of ciphertext

until an intelligible translation into plaintext is obtained.

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Brute-force Attack

Trying all possible key

Tools:

DaveGrohl, Hashcat, Cain and Abel, Hydra, Rainbowcrack

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SUBSTITUTION TECHNIQUES

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

Caesar cipher

The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.

plain: meet me after the toga party

�cipher: PHHW PH DIWHU WKH WRJD SDUWB �

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

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Three important characteristics of this problem enabled us to use a bruteforce

cryptanalysis:

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.

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Monoalphabetic Ciphers �

A permutation of a finite set of elements S 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

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.

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A powerful tool is to look at the frequency of two-letter combinations, known

as digrams.

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PlayFair Ciphers �

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. �

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PlayFair Ciphers Rules �

Digrams
Repeated letters in a pair are split with x (e.g., balloon → ba lx lo on).
If both letters are in the same row, replace each with the next right (wrap around). Example: ar → RM.
If both are in the same column, replace each with the next below (wrap around). Example: mu → CM.
Otherwise, replace each with the letter in its row and the other’s column. Example: hs → BP, ea → IM/JM.

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Example:

Plaintext: attack Ciphertext: rssrde

Plaintext: balloon Ciphertext: ?

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Hill Cipher

Multi-letter cipher
Developed by Hill in 1929
Encrypts a group of letters:digraph , trigraph or polygraph

Concepts to be known:

Matrix arithmetic modulo 26
Square matrix
Determinant
Multiplicative inverse

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CONCEPTS FROM LINEAR ALGEBRA

We define the inverse M^-1 of a square matrix M by the equation M(M^-1) =M^-1M = I, where I is the identity matrix.

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Find determinant
Find Adjoint
Find Inverse

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THE HILL ALGORITHM

This encryption algorithm takes m successive plaintext letters and substitutes for them m ciphertext letters. The substitution is determined by m linear equations in which each character is assigned a numerical value (a = 0, b = 1, c, z = 25).

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POLYALPHABETIC SUBSITUTION CIPHER

Use different monoalphabetic substitutions as one proceeds through the plaintext message.

Techniques have the following features in common:

1. A set of related monoalphabetic substitution rules is used.

2. A key determines which particular rule is chosen for a given transformation.

VIGENÈRE CIPHER

VERNAM CIPHER

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A general equation of the encryption process is

Ci = (pi + ki mod m) mod 26

Decryption is a generalization of equation:

pi = (Ci - ki mod m) mod 26

keyword : DECEPTIVE,

Plaintext: We are discovered save yourself

VIGENÈRE CIPHER

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Autokey system

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VERNAM CIPHER

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ONE TIME PAD

Each new message requires a new key of the same length as the new message.

Such a scheme, known as a one-time pad, is unbreakable.

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Steganography.

A plaintext message may be hidden in one of 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

Ex: Simply encrypt correct reading exactly twice

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Some techniques 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 at 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 up in front of a light.

■ Typewriter correction ribbon: Used between lines typed with a black ribbon,

the results of typing with the correction tape are visible only under a strong

light.

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Drawbacks:

Once the system is discovered , it becomes virtually worthless

Note:
Message can be first encrypted and then hidden using steganography

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Block Ciphers and the Data

Encryption Standard

TRADITIONAL BLOCK CIPHER STRUCTURE

Several important symmetric block encryption algorithms in current use are based on a structure referred to as a Feistel block cipher

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Stream Ciphers and Block Ciphers

A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream ciphers are the autokeyed Vigenère cipher and the Vernam cipher

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

used to produce a ciphertext block of equal length.

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Motivation for the Feistel Cipher Structure

DIFFUSION AND CONFUSION

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FEISTEL CIPHER

STRUCTURE

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Feistel network depends on the choice of the following parameters and design features

Block size: Larger block sizes mean greater security but reduced encryption/decryption speed for a given algorithm.
Key size: Larger key size means greater security but may decrease encryption/

decryption speed

Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate security but that multiple rounds offer increasing security. A typical size is 16 rounds.
Subkey generation algorithm: Greater complexity in this algorithm should

Lead to greater difficulty of cryptanalysis.

Round function F: Again, greater complexity generally means greater resistance

to cryptanalysis.

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FEISTEL DECRYPTION ALGORITHM The process of decryption with a Feistel cipher

is essentially the same as the encryption process.

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)

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For the ith iteration of the encryption algorithm,

LEi = REi-1

REi = LEi-1 ⊕F(REi-1, Ki)

Rearranging terms:

REi-1 = LEi

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

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THE DATA ENCRYPTION ST DATA ENCRYPTION STANDARD

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Data Encryption Standard (DES)

Symmetric block cipher

Input : 64 bits
Output: 64 bits
Key size : 64 bits
Subkey size: 56 bits
Round key: 48 bits
No of Rounds: 16 rounds

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DES Encryption Algorithm

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DES EXAMPLE

The Avalanche Effect

A desirable property of any encryption algorithm is that a small change in either the plaintext or the key should produce a significant change in the ciphertext

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THE STRENGTH OF DES

The Use of 56-Bit Keys

With a key length of 56 bits, there are 2^56 possible keys, which is approximately 7.2 * 1016 keys.

brute-force attack appears impractical.

Assuming that, on average, half the key space has to be searched, a single machine performing one DES encryption per microsecond would take more than a thousand

years to break the cipher.

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The Nature of the DES Algorithm

Cryptanalysis is possible by exploiting eight substitution tables, or S-boxes, that are used in each iteration

There is a suspicion that the boxes were constructed in such a way that cryptanalysis is possible for an opponent who knows the weaknesses in the S-boxes

Despite this, no one has so far succeeded in discovering the supposed fatal weaknesses in the S-boxes

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Timing Attacks

A timing attack is one in which information about the key or the plaintext is obtained by observing how long it takes a given implementation to perform decryptions on various ciphertexts.

A timing attack exploits the fact that an encryption or decryption algorithm often takes slightly different amounts of time on different inputs.

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BLOCK CIPHER DESIGN PRINCIPLES

Three critical aspects of block cipher design: the number of rounds, design of the function F, and key scheduling.

Number of Rounds : The greater the number of rounds, the more difficult it is to perform cryptanalysis, even for a relatively weak F.

So that known cryptanalytic efforts require greater effort than a simple brute-force key search attack.

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Design of Function F

F be nonlinear

Algorithm to have good avalanche properties.

The bit independence criterion (BIC), which states that output bits j and k should change independently when any single input bit i is inverted for all i, j, and k.

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Key Schedule Algorithm

Select subkeys to maximize the difficulty of deducing individual subkeys and the difficulty of working back to the main key

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THANK YOU

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