ITC == Introduction To Cryptography

Pseudorandom Number Generation And Stream Cipher

Algorithm Design

• Purpose-built algorithms
  • 專門用來產生 pseudorandom number (bit stream)
  • 最具代表性: RC4
• Algorithms based on existing cryptographic algorithms
  • Symmetric block ciphers
  • Asymmetric ciphers
  • Hash functions and message authentication codes

Pseudorandom Number Generators

• Linear Congruential Generators

X_{n+1} = \left (aX_{n}+c \right ) \mod m  

• Blum Blum Shub Generator (BBS Generator)

Pseudorandom Number Generation Using A Block Cipher

• PRNG Using Block Cipher Modes of Operation
  • CTR Mode
    
  • OFB Mode
    

Stream Ciphers

• A key is input to a pseudorandom bit generator that produces a stream of 8-bit numbers that are apparently random.

• The output, called keystream, is combined one byte at a time with a plaintext stream using the bitwise XOR operation.

RC4

• A stream cipher designed in 1987 by Ron Rivest for RSA security.