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How many padding bits must be added to a message of 100 characters if 8bit ASCII is used for encoding and the block cipher accepts blocks of 64 bits? Suppose that we have a block cipher where n = 64. If there are 10 1’s in the ciphertext, how many trialanderror tests does Eve need to do to recover the plaintext from the intercepted ciphertext in each of the following cases? | 04.2012 Le Trong Ngoc letrongngoc@hui.edu.vn MODERN CIPHERS 1 MODERN CIPHERS 1 MODERN BLOCK CIPHERS 2 MODERN STREAM CIPHERS 1 MODERN BLOCK CIPHERS How many padding bits must be added to a message of 100 characters if 8-bit ASCII is used for encoding and the block cipher accepts blocks of 64 bits? 1 MODERN BLOCK CIPHERS Substitution or Transposition ? To be resistant to exhaustive-search attack, a modern block cipher needs to be designed as a substitution cipher. Suppose that we have a block cipher where n = 64. If there are 10 1’s in the ciphertext, how many trial-and-error tests does Eve need to do to recover the plaintext from the intercepted ciphertext in each of the following cases? a. The cipher is designed as a substitution cipher. b. The cipher is designed as a transposition cipher. 1 MODERN BLOCK CIPHERS Block Ciphers as Permutation Groups A full-size key n-bit transposition cipher or a substitution block cipher can be modeled as a permutation, but their key sizes are . | 04.2012 Le Trong Ngoc letrongngoc@hui.edu.vn MODERN CIPHERS 1 MODERN CIPHERS 1 MODERN BLOCK CIPHERS 2 MODERN STREAM CIPHERS 1 MODERN BLOCK CIPHERS How many padding bits must be added to a message of 100 characters if 8-bit ASCII is used for encoding and the block cipher accepts blocks of 64 bits? 1 MODERN BLOCK CIPHERS Substitution or Transposition ? To be resistant to exhaustive-search attack, a modern block cipher needs to be designed as a substitution cipher. Suppose that we have a block cipher where n = 64. If there are 10 1’s in the ciphertext, how many trial-and-error tests does Eve need to do to recover the plaintext from the intercepted ciphertext in each of the following cases? a. The cipher is designed as a substitution cipher. b. The cipher is designed as a transposition cipher. 1 MODERN BLOCK CIPHERS Block Ciphers as Permutation Groups A full-size key n-bit transposition cipher or a substitution block cipher can be modeled as a permutation, but their key sizes are different: Transposition: the key is élog2n! bits long. Substitution: the key is élog2(2n)! bits long. 1 MODERN BLOCK CIPHERS Components of a Modern Block Cipher A straight P-box is invertible, but compression and expansion P-boxes are not. 1 MODERN BLOCK CIPHERS Components of a Modern Block Cipher : S-Box An S-box (substitution box) can be thought of as a miniature substitution cipher. An S-box may or may not be invertible. In an invertible S-box, the number of input bits should be the same as the number of output bits. 1 MODERN BLOCK CIPHERS Components of a Modern Block Cipher Exclusive-Or An important component in most block ciphers is the exclusive-or operation. Addition and subtraction operations in the GF(2n) field are performed by a single operation called the exclusive-or (XOR). 1 MODERN BLOCK CIPHERS GF(2n) and uses a set of 2n elements. The elements in this set are n-bit words. The five properties of the exclusive-or operation in the GF(2n) field makes this operation a very .
