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Bảo mật thông tin: Các phương pháp mã hóa - phần 2

To discuss product ciphers and distinguish between two classes of product ciphers: Feistel and non-Feistel ciphers. | Bảo mật hệ thống thông tin CÁC PHƯƠNG PHÁP MÃ HÓA PHẦN 2 1/2011 5. Objectives ❏ 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 5 5. Objectives (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 5 5. 5-1 MODERN BLOCK CIPHERS A symmetric-key modern block cipher encrypts an n-bit block of plaintext or decrypts an n-bit block of ciphertext. The encryption or decryption algorithm uses a k-bit key. 5.1.1 Substitution or Transposition 5.1.2 Block Ciphers as Permutation Groups 5.1.3 Components of a Modern Block Cipher 5.1.4 Product Ciphers 5.1.5 Two Classes of Product Ciphers 5.1.6 Attacks on Block Ciphers Topics discussed in this section: 5. Figure 5.1 A modern block cipher 5.1 Continued 5. 5.1 Continued Example 5.1 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? Encoding 100 characters using 8-bit ASCII results in an 800-bit message. The plaintext must be divisible by 64. If | M | and |Pad| are the length of the message and the length of the padding, Solution 5. A modern block cipher can be designed to act as a substitution cipher or a transposition cipher. 5.1.1 Substitution or Transposition To be resistant to exhaustive-search attack, a modern block cipher needs to be .