1. Momina Tariq: 07030619-021 AmbreenSohail: 07030619-007 Data Communications and Networks Cryptography Data communications and networks

2. Topics Cryptography Components Types of cryptography Symmetric-key cryptography Asymmetric-key cryptography Traditional ciphers Substitution cipher Transposition cipher Data Communications and Networks

3. 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. Data Communications and Networks

4. 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 Data Communications and Networks

5. Components involved in Cryptography (cont.) Encryption:Creating cipher text from plaintext with a specific encryption algorithm and an encryption key Data Communications and Networks Decryption:Covering plaintext from cipher text using a specific encryption algorithm and a decryption key

6. Types of Cryptography Symmetric-key cryptography / Secret-key cryptography Asymmetric-key cryptography / Public-key cryptography Data Communications and Networks

7. 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 Data Communications and Networks

8. Symmetric-key cryptography Data Communications and Networks

9. Traditional Ciphers Two sub types of traditional ciphers: • Substitution ciphers • Transposition ciphers Data Communications and Networks

10. 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 Data Communications and Networks

11. 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” Data Communications and Networks

12. Monoalphabetic Substitution Data Communications and Networks

13. 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 Data Communications and Networks

14. 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. Data Communications and Networks

15. Suppose key length (m)=6 Key word (K) = C I P H E R = (2,8,15,7,4,17) Plaintext (P) = thiscryptosystemisnotsecure The Vigenère Cipher(example) Data Communications and Networks Ciphertext (C) = VPXZGIAXIVWPUBTTMJPWIZITWZI

16. 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 Data Communications and Networks

17. 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 Data Communications and Networks

18. 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 Data Communications and Networks

19. Asymmetric-key Cryptography Data Communications and Networks

20. ANY QUESTIONS??? Data Communications and Networks