ビデオ	担当者：森川 博之・山口 英・村井 純 第02回 10/18/2000

Random Cipher Model (2)

• For all keys K and ciphertext C, decipherment DK(C) uniformly distributed over all 2H0N messages
  
  
• Perfect secrecy: cannot be uniquely deciphered from ciphertext ⇒ i.e., np=0 or more than 2
  
  
• Probability of “meaningful” decipherment: 2HN / 2H0N
  
  
• Then, Np= K･2HN / 2H0N =2Hk-ND
  
  
  • Hk : key entropy
    
    
  • D=H0 - H : language redundancy
    
    
• If N < N0=HK / D, then np > 2 ⇒ perfect secrecy
  
  
• N0 : unicity distance (判別距離)
  
  
  • “Unicity" is the least message length that can be uniquely deciphered