Information system security wk3-2

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Information system security wk3-2

  1. 1. IT346 Information System Security Week 3-2: Symmetric Key Cryptography Faculty of Information Technology Page 1
  2. 2. Block Cipher  Block Cipher block ‣ block encrypt block block ciphertext block encryption ‣ full encrypt encrypt Exrw Faculty of Information Technology time part encrypt vjyo gMsQ time encrypt vjyo Page 2
  3. 3. Block Cipher  ‣ Ciphertext plaintext block ‣ Encryption block Faculty of Information Technology ciphertext plaintext block cipher block block probabilistic output plaintext input Page 3
  4. 4. Block Cipher  Mode Block Cipher ‣ Electronic Code Book (ECB) ‣ Cipher Block Chaining (CBC)  Mode Block Cipher Stream Cipher ‣ Cipher Feedback (CFB) ‣ Output Feedback (OFB) Faculty of Information Technology Page 4
  5. 5. Electronic Code Book (ECB) mode     block block block block Faculty of Information Technology encrypt Page 5
  6. 6. Electronic Code Book (ECB) mode Faculty of Information Technology Page 6
  7. 7. Cipher Block Chaining (CBC) mode  ciphertext Encryption ‣ Chain Block ‣ Block Faculty of Information Technology plaintext Input plaintext block Initial Vector (IV) IV IV key XOR encrypt ciphertext key block Page 7
  8. 8. Cipher Block Chaining (CBC) mode Decryption  ‣ Ciphertext key encryption block decrypt XOR IV plaintext block ‣ Decrypt Ciphertext block Ciphertext block plaintext block ‣ block Faculty of Information Technology XOR decrypt plaintext Page 8
  9. 9. Cipher Block Chaining (CBC) mode C0 = IV Ci = EK(Pi XOR Ci-1) C0 = IV Pi = DK(Ci) XOR Ci-1 Faculty of Information Technology Page 9
  10. 10. Cipher Feedback (CFB) mode  ‣ encrypt ‣ block Faculty of Information Technology Page 10
  11. 11. Cipher Feedback (CFB) mode  Encryption ‣ IV encrypt bits shift register cipher IV bit ‣ cipher IV XOR j plaintext block ciphertext block ‣ ciphertext block ‣ IV shift bit j ciphertext block j ‣ Faculty of Information Technology IV j plaintext Page 11
  12. 12. CFB Mode Encryption Decryption Faculty of Information Technology Page 12
  13. 13. Output Feedback (OFB) mode CFB  ‣ IV ‣ Faculty of Information Technology CFB ciphertext encryption OFB ciphertext IV IV Page 13
  14. 14. Output Feedback (OFB) mode Faculty of Information Technology Page 14
  15. 15. Symmetric Key Cryptography Symmetric Key Cryptography key Encryption Decryption ?  Alice Bob Alice Bob Bob Alice Symmetric Key  Faculty of Information Technology Page 15
  16. 16. Diffie-Hellman  Diffie-Hellman (Exponential) (Logarithm) Secret Key  Primitive Modulo p) Faculty of Information Technology g p g<p g p (Primitive Root Page 16
  17. 17. Diffie-Hellman (Example) g = 3, p = 5 Primitive  ‣ ‣ ‣ ‣ ‣ ‣ g1 mod 5 = 3 mod 5 = 3 g2 mod 5 = 9 mod 5 = 4 g3 mod 5 = 27 mod 5 = 2 g4 mod 5 = 81 mod 5 = 1 g5 mod 5 = 243 mod 5 = 3 g6 mod 5 = 729 mod 5 = 4  Faculty of Information Technology 3 primitive 5 Page 17
  18. 18. Diffie-Hellman Secret Key  Alice   g a Bob p  A = ga mod p  g, p, A Bob  B Faculty of Information Technology Bob g, p, A B   b  p  g, p, A B = gb mod Alice K= Page 18
  19. 19. Diffie-Hellman  g = 3 and p = 5 Alice  g=3 p=5 g=3, p  a=2 =5, A  A = 32 mod =4 5=4 B  g=3, p=5, A=4 Bob Faculty of Information Technology Bob  g=3, p=5, A=4  b=3  B = 33 mod 5 =2 Alice  K = 43 Page 19

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