Information system security wk3-2
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Information system security wk3-2
- 1. IT346 Information System Security Week 3-2: Symmetric Key Cryptography Faculty of Information Technology Page 1
- 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. Block Cipher ‣ Ciphertext plaintext block ‣ Encryption block Faculty of Information Technology ciphertext plaintext block cipher block block probabilistic output plaintext input Page 3
- 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. Electronic Code Book (ECB) mode block block block block Faculty of Information Technology encrypt Page 5
- 6. Electronic Code Book (ECB) mode Faculty of Information Technology Page 6
- 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. 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. 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. Cipher Feedback (CFB) mode ‣ encrypt ‣ block Faculty of Information Technology Page 10
- 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. CFB Mode Encryption Decryption Faculty of Information Technology Page 12
- 13. Output Feedback (OFB) mode CFB ‣ IV ‣ Faculty of Information Technology CFB ciphertext encryption OFB ciphertext IV IV Page 13
- 14. Output Feedback (OFB) mode Faculty of Information Technology Page 14
- 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. 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. 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. 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. 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