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# 3-DES - Modern Block Ciphers CSE 651 Introduction to...

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Modern Block Ciphers CSE 651: Introduction to Network Security

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Summary Block Ciphers (Chapter 3) Feistel Cipher Structure (Chapter 3) DES: Data Encryption Standard (Ch. 3) 3DES (Ch 6.1) AES: Advanced Encryption Standard (Ch. 5.2) 2
Monoalphabetic Substitution Cipher Shuffle the letters and map each plaintext letter to a different random ciphertext letter: Plain letters: a b c defghijklmnopqrstuvwxyz Cipher letters: D K V QFIBJWPESCXHTMYAUOLRGZN Plaintext: if we wish to replace letters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA What does a key look like? 3

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Playfair Key Matrix Use a 5 x 5 matrix. Fill in letters of the key (w/o duplicates). Fill the rest of matrix with other letters. E.g., key = MONARCHY. M O N A R C H Y B D E F G I/J I/J K L P Q S T U V W X Z 4
Vigenère Cipher Simplest polyalphabetic substitution cipher Consider the set of all Caesar ciphers: { C a , C b , C c , ..., C z } Key: e.g. security Encrypt each letter using C s , C e , C c , C u , C r , C i , C t , C y in turn. Repeat from start after C y . 5

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Basic idea of modern block ciphers From classical ciphers, we learn two techniques that may improve security: Encrypt multiple letters at a time Use multiple ciphertext alphabets (Polyalphabetic ciphers) Combining these two techniques encrypt eight (or more) letters at a time called a block cipher and use an extremely large number of ciphertext alphabets will be called modes of operation 1
Block Ciphers In general, a block cipher replaces a block of N plaintext bits with a block of N ciphertext bits. (E.g., N = 64 or 128.) A block cipher is a monoalphabetic cipher. Each block may be viewed as a gigantic character . The “alphabet” consists of 2 N gigantic characters. Each particular cipher is a one-to-one mapping from the plaintext “alphabet” to the ciphertext “alphabet”. There are 2 N ! such mappings. A secret key indicates which mapping to use. 7

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Ideal Block Cipher An ideal block cipher would allow us to use any of these 2 N ! mappings. The key space would be extremely large. But this would require a key of log 2 (2 N !) bits. If N = 64, log 2 (2 N !) ≈ N x 2 N ≈ 10 21 bits ≈ 10 11 GB. Infeasible! 8
Practical Block Ciphers Modern block ciphers use a key of K bits to specify a random subset of 2 K mappings. If K ≈ N, 2 K is much smaller than 2 N ! But is still very large. If the selection of the 2 K mappings is random , the resulting cipher will be a good approximation of the ideal block cipher. Horst Feistel, in1970s, proposed a method to achieve this. 9

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The Feistel Cipher Structure Input: a data block and a key Partition the data block into two halves L and R. Go through a number of rounds.
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3-DES - Modern Block Ciphers CSE 651 Introduction to...

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