Cryptography

CRYPTOGRAPHY

Ms. Date SarojME CSE (Part Time)

CRYPTOGRAPHY comes from the Greek words for “ secret writing”.

It is the science of devising methods that allow information to be sent in a secure form in such a way that the only person, who possess the key can read it.

It is the collection of algorithm & associated procedures for hiding & unhiding information

Plaintext :Messages to be encrypted.

Cipher text: Output of encryption process.

Key

Intruder( Active & Passive)

Channel

Encryption Model, E

Encryption Model, E

ChannelChannel Decryption Model, D

Decryption Model, D

Introduction to CryptographyIntroduction to Cryptography

EkEk DKDK

IntruderIntruder

pLAINTEXT

pLAINTEXT

PLAINTEXT

PLAINTEXTCIPHERTEXTCIPHERTEXT

ActiveIntruderActive

IntruderPassive IntruderPassive Intruder

FIG. THE ENCRYPTION MODEL (For a Symmetric-Key Cipher)

The messages to be encrypted ,known as Plaintext, are transformed by a key.

The output of encryption process, known as Cipher text, is the transmitted through the channel.

We assume that the intruder hears & accurately copies down the complete Cipher text. However, unlike the intended receiver, the intruder does not know what the decryption key is and so can’t decrypt the Cipher text easily.

CryptographyCryptography

CryptographyCryptography Passive intruders can only listen to the

communication channel Active intruders can modify the

legitimate messages before they get to the receiver.

Cryptanalysis: The art of breaking ciphers. Cryptography: The art of devising ciphers. & Cryptology means -----------

CRYPTOLOGYCRYPTOLOGY CRYPTANALYSISCRYPTANALYSIS CRYPTOGRAPHYCRYPTOGRAPHY== ++

It will be often useful to have a notation for relating plaintext, cipher text & Keys.

Let’s assume that ,the plaintext has n characters , drawn from an alphabet X.

We represent the plaintext msg. as a vector of n symbols— X={x1,x2,x3,……xn}

Before transmitting, the sender Encrypts the msg.

This is done by a rule that maps X into a sequence of new characters called as Cipher text.

Y={y1,y2,y3,…..,yn}

Notation:Notation:

The mapping from X to Y is based on an encryption rule ,which we may write as

Y=Ek (X) The subscript k (Key) denotes a particular

rule, employed in a ciphersystem to determine the cipher text.

The cipher text msg. is transmitted to the receiver.

In order to read the original msg.,the receiver must use another rule ,called a decryption rule.

Notation: Notation: Contd…..Contd…..

This decryption rule can be looked as an inverse mapping defined by encryption rule.

X=Dk (Y)

Notation: Notation: Contd…….Contd…….

Y = Ek (X)Y = Ek (X)

X = Dk (Y)X = Dk (Y)X =

Dk(Ek(X))X =

Dk(Ek(X))

Let’s look at the methods by which cryptosystems can be attacked:

1. Cipher text-only attack 2. Known plaintext attack. 3. Chosen plaintext attack 4. Chosen cipher text attack.

Regardless of any attack intruder’s task is to determine the key. If the key is known, the cryptosystem is broken.

Encryptions methods have historically been divided into two categories:

1. Substitution Ciphers 2.Transposition Ciphers

In a substitution cipher each letter or group of letters is replaced by another letter or group of letters.

One of the oldest known ciphers is the Caesar Cipher. In this method a becomes D, b becomes E,….. , z becomes C.

For example attack becomes DWWDFN.

A generalization is shift cipher text alphabets by k letters.

Transposition Ciphers Substitution ciphers preserves the order of

alphabets. Transposition ciphers reorder the letters.

Some simple cipher systems

1. The shift Cipher 2. The Vigenere Cipher 3. The permutation Cipher 4. The Substitution Cipher

Codes It is also possible to increase the security of

encryption by encrypting the entire words, phrases, sentences, before the applying to the encryption model.

Secret codebook is shared between sender & receiver.

Even if the cipher has been broken , Intruder will still face the problem of decoding it.

Provides data compression to the plaintext.

Codes contd….

The combination of coding & ciphering, if used properly, can produce a cryptosystem that is far more difficult to break than a cryptosystem that uses either.

DISADVANTAGE To prevent from the attacks, the codebook

needs to be changed frequently. But changing & securely communicating codebooks is difficult challenge. So, codes are not well adapted in modern communication systems.

THANK YOU !........