Cryptography Data communications and networks Momina Tariq: Ambreen Sohail: Data Communications and Networks

2 T OPICS Cryptography Components Types of cryptography Symmetric-key cryptography Asymmetric-key cryptography Traditional ciphers Substitution cipher Transposition cipher

Data Communications and Networks 3 C RYPTOGRAPHY 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 C OMPONENTS INVOLVED IN C RYPTOGRAPHY 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 C OMPONENTS INVOLVED IN C RYPTOGRAPHY ( 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

Data Communications and Networks 6 T YPES OF C RYPTOGRAPHY Symmetric-key cryptography / Secret-key cryptography Asymmetric-key cryptography / Public-key cryptography

Data Communications and Networks 7 S YMMETRIC - 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 S YMMETRIC - KEY CRYPTOGRAPHY

9 T RADITIONAL C IPHERS Two sub types of traditional ciphers: Substitution ciphers Transposition ciphers

Data Communications and Networks 10 S UBSTITUTION 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 M ONOALPHABETIC S UBSTITUTION “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 M ONOALPHABETIC S UBSTITUTION

Data Communications and Networks 13 P OLYALPHABETIC C IPHERS 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 T HE V IGENÈRE C IPHER 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.

T HE V IGENÈRE C IPHER ( EXAMPLE ) Pthiscryptosy K CVPXZGIAXIVWP PstemisnotsecuRe K CUBTTMJPWIZITWZT 15 Data Communications and Networks Suppose key length (m)=6 Key word (K) = C I P H E R = (2,8,15,7,4,17) Plaintext (P) = thiscryptosystemisnotsecure Ciphertext (C) = VPXZGIAXIVWPUBTTMJPWIZITWZI

Data Communications and Networks 16 T HE V IGENÈRE T ABLE 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 T RANSPOSITION C IPHER 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 A SYMMETRIC -K EY C RYPTOGRAPHY 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 A SYMMETRIC - KEY C RYPTOGRAPHY

Data Communications and Networks 20 ANY QUESTIONS ? ? ?