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

Secure Cryptographic Systems

• Unconditionally secure: achieved if an interceptor cannot recover plaintext from ciphertext regardless of the time expended
  
  
• Shannon’s bold definition
  
  
  • A secrecy system provides “perfect secrecy” against a ciphertext-only attack if
    
    
    H(M|E) = H(M)
    
    
    which is equivalent to saying that M and E are statistically independent
    
    
• This was the first precise statement of what it means to say that a secrecy system is “unbreakable”