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

Cryptanalysis

• The science of recovering the plaintext of a message without access to the key
  
  
• Ciphertext-only attack
  
  
  • Exhaustive search until “recognizable plaintext” 　⇒　need enough ciphertext
    
    
• Know-plaintext attack
  
  
  • The attacker obtain <plaintext, ciphertext> pairs
    
    
• Chosen-plaintext attack
  
  
  • The attacker can choose what plaintext to encrypt
    
    
• A cryptosystem should resist all three sorts of attacks