CryptographyData communications and networks

Momina Tariq: 07030619-021Ambreen Sohail: 07030619-007

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TOPICSCryptographyComponentsTypes of cryptography

Symmetric-key cryptographyAsymmetric-key cryptography

Traditional ciphersSubstitution cipherTransposition cipher

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CRYPTOGRAPHY

Cryptography is the practice and study of hiding information.

A field of mathematics and computer science concerned with information security and related issues, particularly encryption and authentication.

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COMPONENTS INVOLVED IN CRYPTOGRAPHY

Plaintext - Text message to which an algorithm is applied

Cipher text - Encrypted or scrambled message produced by applying the algorithm to the plaintext message using the secret key

Cipher - A message written in a secret code

Key - A key is a number or set of numbers that the cipher, as an algorithm, operates on

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COMPONENTS INVOLVED IN CRYPTOGRAPHY (CONT.)

Encryption: Creating cipher text from plaintext with a specific encryption algorithm and an encryption key

Decryption: Covering plaintext from cipher text using a specific encryption algorithm and a decryption key

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TYPES OF CRYPTOGRAPHY

Symmetric-key cryptography / Secret-key cryptography

Asymmetric-key cryptography / Public-key cryptography

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SYMMETRIC-KEY CRYPTOGRAPHY

An encryption system in which the sender and receiver of a message share a single, common key that is used to encrypt and decrypt the message

Symmetric-key cryptography is sometimes called secret-key cryptography

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SYMMETRIC-KEY CRYPTOGRAPHY

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TRADITIONAL CIPHERS

Two sub types of traditional ciphers:

Substitution ciphers

Transposition ciphers

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SUBSTITUTION CIPHERS

“A substitution cipher substitutes one symbol with another. If the symbols in the plaintext are alphabetic characters, we replace one character with another.”

Types of substitution ciphers

Monoalphabetic cipher

Polyalphabetic cipher

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MONOALPHABETIC SUBSTITUTION

“In a monoalphabetic cipher, a character (or symbol) in the plaintext is always changed to the same character(or symbol) in the ciphertext regardless of its position in the text. It simply means one-to-one relationship”

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MONOALPHABETIC SUBSTITUTION

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POLYALPHABETIC CIPHERS

A cipher that uses multiple substitution alphabets is called a polyalphabetic substitution cipher.

The Vigenere Cipher and Hill Cipher are examples of alphabetic ciphers

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THE VIGENÈRE CIPHER The Vigenère Cipher makes use of a

keyword of length m.

So, First step: determine the key length=m,

Second step: determine the key (word) itself

After that encryption/decryption of the

message is easy.

THE VIGENÈRE CIPHER(EXAMPLE)

P t h i s c r y p t o s y

19 7 8 18 2 17 24 15 19 14 18 24

K 2 8 15 7 4 17 2 8 15 7 4 17

21 15 23 25 6 8 0 23 8 21 22 15

C V P X Z G I A X I V W P

P s t e m i s n o t s e c u R e

18 19 4 12 8 18 13 14 19 18 4 2 20 17 4

K 2 8 15 7 4 17 2 8 15 7 4 17 2 8 15

20 1 19 19 12 9 15 22 8 25 8 19 22 25 19

C U B T T M J P W I Z I T W Z T

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Suppose key length (m)=6Key word (K) = C I P H E R = (2,8,15,7,4,17) Plaintext (P) = thiscryptosystemisnotsecure

Ciphertext (C) = VPXZGIAXIVWPUBTTMJPWIZITWZI

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THE VIGENÈRE TABLE 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

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 A C 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 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 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 D F G H I J K L M N O P Q R S T U V W X Y Z A B C D E G H I J K L M N O P Q R S T U V W X Y Z A B C D E F H I J K L M N O P Q R S T U V W X Y Z A B C D E F G I J K L M N O P Q R S T U V W X Y Z A B C D E F G H J K L M N O P Q R S T U V W X Y Z A B C D E F G H I K L M N O P Q R S T U V W X Y Z A B C D E F G H I J L M N O P Q R S T U V W X Y Z A B C D E F G H I J K M N O P Q R S T U V W X Y Z A B C D E F G H I J K L N O P Q R S T U V W X Y Z A B C D E F G H I J K L M O P Q R S T U V W X Y Z A B C D E F G H I J K L M N P Q R S T U V W X Y Z A B C D E F G H I J K L M N O Q R S T U V W X Y Z A B C D E F G H I J K L M N O P R S T U V W X Y Z A B C D E F G H I J K L M N O P Q S T U V W X Y Z A B C D E F G H I J K L M N O P Q R T U V W X Y Z A B C D E F G H I J K L M N O P Q R S U V W X Y Z A B C D E F G H I J K L M N O P Q R S T V W X Y Z A B C D E F G H I J K L M N O P Q R S T U W X Y Z A B C D E F G H I J K L M N O P Q R S T U V X Y Z A B C D E F G H I J K L M N O P Q R S T U V W Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Z 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

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TRANSPOSITION CIPHER

Methods of encryption by which the positions held by units of plaintext are shifted according to a regular system

For example: Plaintext: V O Y A G E R Ciphertext: O V A Y E G R where every two letters are switched with

each other

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ASYMMETRIC-KEY CRYPTOGRAPHY

A pair of keys is used. The sender uses the public key; the receiver uses the private key.

Asymmetric-key cryptography is sometimes called Public-key cryptography

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ASYMMETRIC-KEY CRYPTOGRAPHY

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