tailieunhanh - MODERN 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. | MODERN CIPHERS 1 MODERN BLOCK CIPHERS 2 MODERN STREAM CIPHERS 1 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. 2 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 r log2n bits long. Substitution the key is r log2 2n l 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.