Basic Cryptography 1

Basic Cryptography 1：Basic concept、Terminologies、Type of encryption schemes……

Basic Cryptography 1

• Basic concept
  • Crytography
    • the study of transforming a plaintext into a ciphertext and transfer backwards
  • Cryptanalysis
    • The study of transforming a ciphertext back into the original plaintext without "Key"
• Terminologies
  • Plaintext
    • the original clear message
  • Ciphertext
    • the coded message to protect the plaintext
  • Key
    • a data unit used for encrypting the plaintext/decrypting the ciphertext
    • should be known only by sender & receiver
  • Cipher
    • an algorithm for performing encryption or decryption
  • Encipher/Encrypt
    • the process of converting plaintext to ciphertext using a cipher and a key
  • Decipher/Decrypt
    • the process of converting ciphertext back into plaintext using a cipher and a key
• Type of encryption schemes
  • symmetric key encryption
  • asymmetric key encrytion
    • AKA. public-key encryption
    • APP
      • Key distribution
      • digital signatures
• Brief history
  • non-scientific cryptography
    • from antiquity until 1949
  • scientific crytography
    • starts with Shannon's paper
      • information theory (1948)
      • communication theory of secrecy systems(1949)
  • "New directions in crytography", Diffie& Hellman,1976, Explosion in crytologic research.
    • show secret communication is possible without a shared key
• Type of Classical Ciphers
  • Substitution Ciphers
    • Caesar
      • replace each letter by KEY letter on
    • monoalphabetic
      • each plaintext letter maps to a different random ciphertext letter
    • Classical cipher
    • polyalphabetic
      • DECEPTIVEDECEPTIVEDECEPTIVE......
    • playfair
    • Autokey
  • Transposition
    • Rail Fence
    • Row Transposition
• Security of Classical Ciphers
  • Statistical Cryptanalysis
    • frenquency of characters are important clue
      • English Cipher frequency
      • Frequent bigrams
      • Frequent trigrams
• One-Time Pad
  • truly radom key
  • as long as the message
  • disadvantage
    • can only use the key once, have problem of safe distribution of key
• Product Ciphers
  • using several ciphers insuccession to make harder
  • Modern Cipher
• Steganography
  • Hide existence of data
  • Disadvantage
    • High Overhead