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Cns Chapter 2
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Cns Chapter 2

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Transcript

  • 1. Classical encryption techniques
  • 2. Science of secret-overview
  • 3. Conventional Encryption Principles
    • An encryption scheme has five ingredients:
      • Plaintext
      • Encryption algorithm
      • Secret Key
      • Ciphertext
      • Decryption algorithm
    • Security depends on the secrecy of the key, not the secrecy of the algorithm
  • 4. Conventional encryption requirements
    • Strong algorithm
      • For known plain-cipher text pairs, opponets should not be able to decrypt cipher text or the key
    • Key secrecy
      • Sender and receiver should receive
      • Store
  • 5. Conventional Encryption Principles
  • 6. Cryptography
    • Classified along three independent dimensions:
      • The type of operations used for transforming plaintext to ciphertext
      • The number of keys used
        • symmetric (single key)
        • asymmetric (two-keys, or public-key encryption)
      • The way in which the plaintext is processed
        • Block or stream
  • 7. Attacks on conventional encryption system
    • Cryptanalysis
      • Attacks rely on nature of algorithm
      • Knowledge of plaintext characteristics
    • Brute force attacks
      • Attack by all combinations of key
      • On an average half of possible keys
  • 8. Average time required for exhaustive key search 2.15 milliseconds 2 32 = 4.3 x 10 9 32 5.9 x 10 30 years 2 168 = 3.7 x 10 50 168 5.4 x 10 18 years 2 128 = 3.4 x 10 38 128 10 hours 2 56 = 7.2 x 10 16 56 Time required at 10 6 Decryption/ ┬Ás Number of Alternative Keys Key Size (bits)
  • 9. Types of cryptanalysis
    • Cipher text only, algorithm known
      • Brute force impractical for large keys
      • Must rely on type of plain text
    • Known Plain and cipher text pairs
      • Pattern of plain text
      • Deduce transformation and derive key
    • Chosen plain text
      • Introduce chosen patterns for encryption
    • Chosen cipher text
    • Chosen text