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UND CSCI 389 - Introduction to Modern Symmetric-key Ciphers

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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60Slide 61Slide 62Slide 63Slide 64Slide 65Slide 66Slide 67Slide 68Slide 69Slide 70Slide 71Slide 72Slide 73Slide 74Slide 75Slide 76Slide 775.1Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Chapter 5Introduction to Modern Symmetric-keyCiphers5.2Objectives ❏ To distinguish between traditional and modern symmetric-key ciphers. ❏ To introduce modern block ciphers and discuss their characteristics. ❏ To explain why modern block ciphers need to be designed as substitution ciphers. ❏ To introduce components of block ciphers such as P-boxes and S-boxes.Chapter 55.3Objectives (Continued) ❏ To discuss product ciphers and distinguish between two classes of product ciphers: Feistel and non-Feistel ciphers. ❏ To discuss two kinds of attacks particularly designed for modern block ciphers: differential and linear cryptanalysis. ❏ To introduce stream ciphers and to distinguish between synchronous and nonsynchronous stream ciphers. ❏ To discuss linear and nonlinear feedback shift registers for implementing stream ciphers.Chapter 55.45-1 MODERN BLOCK CIPHERS5-1 MODERN BLOCK CIPHERSA symmetric-key modern block cipher encrypts an A symmetric-key modern block cipher encrypts an n-bit block of plaintext or decrypts an n-bit block of n-bit block of plaintext or decrypts an n-bit block of ciphertext. The encryption or decryption algorithm ciphertext. The encryption or decryption algorithm uses a k-bit key. uses a k-bit key. 5.1.1 Substitution or Transposition5.1.2 Block Ciphers as Permutation Groups5.1.3 Components of a Modern Block Cipher5.1.4 Product Ciphers5.1.5 Two Classes of Product Ciphers5.1.6 Attacks on Block CiphersTopics discussed in this section:Topics discussed in this section:5.5Figure 5.1 A modern block cipher5.1 Continued5.65.1 ContinuedExample 5.1How many padding bits must be added to a message of 100 How many padding bits must be added to a message of 100 characters if 8-bit ASCII is used for encoding and the block characters if 8-bit ASCII is used for encoding and the block cipher accepts blocks of 64 bits?cipher accepts blocks of 64 bits?Encoding 100 characters using 8-bit ASCII results in an 800-Encoding 100 characters using 8-bit ASCII results in an 800-bit message. The plaintext must be divisible by 64. If | M | and bit message. The plaintext must be divisible by 64. If | M | and |Pad| are the length of the message and the length of the |Pad| are the length of the message and the length of the padding,padding,SolutionSolution5.7A modern block cipher can be designed to act as a substitution cipher or a transposition cipher. 5.1.1 Substitution or TranspositionTo be resistant to exhaustive-search attack, a modern block cipher needs to bedesigned as a substitution cipher.Note5.8Example 5.25.1.1 ContinuedSuppose that we have a block cipher where Suppose that we have a block cipher where nn = 64. If there = 64. If there are 10 1’s in the ciphertext, how many trial-and-error tests are 10 1’s in the ciphertext, how many trial-and-error tests does Eve need to do to recover the plaintext from the does Eve need to do to recover the plaintext from the intercepted ciphertext in each of the following cases?intercepted ciphertext in each of the following cases? a. The cipher is designed as a substitution cipher.a. The cipher is designed as a substitution cipher. b. The cipher is designed as a transposition cipher.b. The cipher is designed as a transposition cipher.a.a.In the first case, Eve has no idea how many 1’s are in the In the first case, Eve has no idea how many 1’s are in the plaintext. Eve needs to try all possible 2plaintext. Eve needs to try all possible 26464 64-bit blocks to 64-bit blocks to find one that makes sense. find one that makes sense. SolutionSolutionb.b.In the second case, Eve knows that there are exactly 10 1’s In the second case, Eve knows that there are exactly 10 1’s in the plaintext. Eve can launch an exhaustive-search in the plaintext. Eve can launch an exhaustive-search attack using only those 64-bit blocks that have exactly 10 attack using only those 64-bit blocks that have exactly 10 1’s. 1’s.5.9Is a modern block cipher a group? 5.1.2 Block Ciphers as Permutation GroupsFull-Size Key Transposition Block CiphersIn a full-size key transposition cipher We need to have n! possible keys, so the key should have log2 n! bits.Example 5.3Show the model and the set of permutation tables for a 3-bit Show the model and the set of permutation tables for a 3-bit block transposition cipher where the block size is 3 bits.block transposition cipher where the block size is 3 bits.SolutionSolutionThe set of permutation tables has 3! = 6 elements, as shown in The set of permutation tables has 3! = 6 elements, as shown in Figure 5.2. Figure 5.2.5.10Figure 5.2 A transposition block cipher modeled as a permutation5.1.2 Continued5.11Full-Size Key Substitution Block CiphersA full-size key substitution cipher does not transpose bits; it substitutes bits. We can model the substitution cipher as a permutation if we can decode the input and encode the output. 5.1.2 ContinuedExample 5.4Show the model and the set of permutation tables for a 3-bit Show the model and the set of permutation tables for a 3-bit block substitution cipher.block substitution cipher.Figure 5.3 shows the model and the set of permutation tables. Figure 5.3 shows the model and the set of permutation tables. The key is also much longer, The key is also much longer, loglog2240,32040,320= 16= 16 bits. bits. SolutionSolution5.12Figure 5.3 A substitution block cipher model as a permutation5.1.2 Continued5.135.1.2 ContinuedA full-size key n-bit transposition cipher or a substitution block cipher can be


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UND CSCI 389 - Introduction to Modern Symmetric-key Ciphers

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