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Cryptography- An Introduction

Cryptography- An Introduction - ComputerSecurity 1...

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11/07/11 1 Computer Security Cryptography –an introduction

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11/07/11 2 Encryption key K E key K D x plaintext y ciphertext original plaintext x . encryption decryption Eavesdropper
11/07/11 3 Encryption A cryptosystem involves an encryption algorithm E , and a a decryption algorithm D Both algorithms make use of a key. Let K E be the encryption key and K D the decryption key. For symmetric cryptosystems the same key is used both encryption and decryption: K E = K D .

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11/07/11 4 Encryption If P is the plaintext message, C the ciphertext, then for symmetric cryptosystems: C = E K ( P ) and P = D K ( E K ( P )) = D K ( C ) For an asymmetric cryptosystem C = E K E ( P ) and P = D K D ( E K E ( P )) = D K D ( C )
11/07/11 5 Kerchoffs’ assumption The adversary knows all details of the encrypting function except the secret key

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11/07/11 6 Symmetric key encryption There are two types of cipher systems: Stream ciphers, Block ciphers.
11/07/11 7 Stream ciphers Encryption x = ISSOPMI …                   y = wdhuvad Key K E

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11/07/11 8 Block ciphers Encryption x = XNEMT OIGNA TPHPM YRKRN y = . Key K E wdmar . hutpkw vaptrh dgdsct
11/07/11 9 Cryptanalysis  Attacks on Cryptosystems Ciphertext only attack: the opponent possesses a string of ciphertexts: y 1 , y 2 , … Known plaintext attack: the opponent possesses a string of plaintexts x 1 , x 2 , … and the corresponding string of ciphertexts: y 1 , y 2 , …

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11/07/11 10  Attacks on Cryptosystems Chosen plaintext attack: the opponent can choose a string of plaintexts x 1 , x 2 , … and obtain the corresponding string of ciphertexts: y 1 , y 2 , … Chosen ciphertext attack: the opponent can choose a string of ciphertexts: y 1 , y 2 , … and get the corresponding string of plaintexts x 1 , x 2 , … Brute force attack: exhaustively, for a given plaintext x and ciphertext y try encrypting x with all possible keys until you get the ciphertext y .
11/07/11 11  Attacks on Cryptosystems In all these attacks the goal of the adversary is to decrypt a challenge ciphertext.

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11/07/11 12 Cryptanalysis  (breaking cryptosystems) Ciphertext attack – the traditional way was to use the statistical properties of the language. Most of the classical ciphers are broken this way. Known plaintext attack Linear Cryptanalysis, see DES. Chosen plaintext attack – Differential cryptanalysis, see DES.
11/07/11 13 Block ciphers An overview of the DES Algorithm DES is an iterated block cipher with 16 rounds, block length 64 bits and key length 56 bits

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11/07/11 14 Iterating Block ciphers* 1. Iterated block cipher Random (binary) key K round keys : K 1 ,…, K Nr , 2. Round function  g w r = g ( w r-1 , K r ), where w r-1 is the previous state
11/07/11 15 Iterated cipher …* Encryption operation : w 0 x ( x = plaintext ) w 1 = g(w 0 , K 1 ), w 2 = g(w 1 , K 2 ), w Nr = g(w Nr- 1 , K Nr ), y w Nr ( y = ciphertext )

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11/07/11 16 Iterated cipher …*
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Cryptography- An Introduction - ComputerSecurity 1...

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