Basic Cryptography 2

modern cryptography ，有Modern Symmetric Ciphers、Stream Ciphers、ChaCha20、BlockCipher...

Basic Cryptography 2

• Modern Symmetric Ciphers
  • deal with info in binary bit level
  • 2 major approaches
    • Stream Ciphers
    • Block Ciphers
• Stream Ciphers
  • key length 126bits,256 bits...
  • Maximum plaintext length: arbitrary long
  • synchronous V.S. asynchronous
  • security
    • secret key cannot be derived
    • the subsequenct segment of the keystream cannot be derived
• ChaCha20
  • intro
    • by D.J. Berstein (2008)
      • refinement of Salsa20
    • Use a 256-bit key
      • 8 * 32 bit
    • other input
      • 4 * 32 bit constants
      • 2 * 32 bit a nonce
      • 2 * 32 bit a block counter
  • 20 rounds
    • quarter round
    • (column0123 + diagonal 1234)* 10
  • design consideration
    • stream cipher eventually repeats
      • should have a large period
        • ChaCha20 2-word counter allow this
    • approximate the properties of a true random number stream as close as possible
      • ChaCha20, quarter-round functions allow high deffusion
    • Key should be long enough to resist brute-force atttack
      • ChaCha20 256-bits KEY
• BlockCipher
  • brief
    • msg broke into fixed sized blocks
    • encrypted one block at a time
  • blocksize choice
    • small block size may be insecure
    • 前后plaintext一样Ciphertext不一样
  • Block cipher principles
    • most symmetic block ciphers are based on Feistel Cipher Structure
    • Block cipher looks like a large substitution
    • create ciphertext from smaller building blocks
• Claude Shannon & Substitution-Permutation Cipher
  • Shannon introduced the idea of substitution-permutation networks(1949)
  • form the basis of modern block ciphers
  • S-P network based on 2 prmitive cryptographics operations
    • substitution S-box
    • permutation P-box
  • provide confusion and diffusion of msg
    • completely obsecure statistical properties of original msg
    • Shannon suggest combining elements to obtain
      • Diffusion
        • 子主题
      • confusion
    • every bit in cipher text should depend on as many bits as possible in the paintext and the key
• Feistel Cipher
  • structure
    • Horst Feistel (IBM)
    • partition input blocks into two halves
    • implement Shannon's substitution-permutation network concept
  • design principle
    • Block size
      • improve block size, security ↑, decrypt speed↓
    • Key size
      • improve key size, security ↑, decrypt speed↓
    • number of rounds
      • improve round#,security ↑, decrypt speed↓
    • subkey gerneration
      • complexity ↑,security ↑, decrypt speed↓
    • round function
      • complexity ↑,security ↑, decrypt speed↓
    • fast software en/decryption & ease of analysis
      • recent concern for practical use
• Data Encryption Standard (DES)
  • block cipher
    • both plaintext and cipher text 64 bits
    • KEY 64 bit (56-bit effective key)
  • input of DES
    • data broke into 64-bit blocks(add pad if necessary)
    • KEY (56-bit effective key + 8-bit parity bits)
  • DES Flow
    • 64-bits plantext
    • initial permutation
    • broke into right half(32-bits) and left half(32-bits)
    • 16 rounds of identical operations
      • data combined with 16 keys of 48-bits(derived from the 56-bit effective key)
    • L & R joined
    • final permutation
    • repeative
      • readily implementable in hardware and software
  • DES decryption
    • almost the same except that 16 sub-keys are entered in reverse order
  • subkey gerneration
    • odd parity
  • security of DES
    • recommanded minimum key size of any symmetric encryption schemes is 112-bits (80-bits for legacy system)
      • to give a better security margin, key size recommended to be at least 128-bits
• Modes of Operations
  • Electronic codebook mode (ECB)
  • Cipher Block Chaining mode (CBC)
  • Cipher feedback mode (CFB)
  • Output feedback mode (OFB)
  • Counter mode (CTR)