Information system security it346 wk4-1
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Information system security it346 wk4-1 Presentation Transcript

  • 1. IT346 Information System Security Week 4-1: Stream Cipher & Asymmetric Key Cryptography Faculty of Information Technology Page 1
  • 2. Symmetric Key Cryptography Cryptography  Symmetric Key Plaintext (Block Cipher) ‣ • DES: • 3DES: • AES: 1 1 Data Block = 64 bits 1 Data Block = 64 bits 1 Data Block = 128 bits (Stream Cipher) ‣ Faculty of Information Technology Page 2
  • 3. Stream Ciphers bit  ‣  Key ‣ Keystream bit (Stream) Keystream Stream Cipher pseudorandom keystream ‣ Pseudorandom Faculty of Information Technology Page 3
  • 4. Stream Ciphers bit  (   Keystream XOR) bit random keystream plaintext plaintext Keystream C = P XOR plaintext StreamKey Key encrypt i Faculty of Information Technology i i Page 4
  • 5. Stream Ciphers  Secret Key Seed Keystream Key Stream Cipher Pseudorandom K Key K KeyStream Generator (Pseudorandom byte generator) KeyStream Generator (Pseudorandom byte generator) Plaintext Byte Stream P + ENCRYPTION Faculty of Information Technology Ciphertext Byte Stream C + Plaintext Byte Stream P DECRYPTION Page 5
  • 6. One-Time-Pad (OTP)  Stream Cipher One-Time-Pad Cipher break ‣ Keystream Vernam (unbreakable cipher) random number Secret Key Pseudorandom number generator ‣ Secret Key OTP Keystream plaintext OTP Faculty of Information Technology OTP Page 6
  • 7. Stream Cipher Properties  Encryption ‣ ‣ Keystream random Stream Cipher random ‣ Secret Key Brute-force Attack bits  Faculty of Information Technology Page 7
  • 8. RC4  RC4 Stream Cipher Ron Rivest RSA Security (Security Company)  Key (variable key size) (Byte-oriented Stream Cipher) random permutation  RC4 SSL/TLS wireless WEP Faculty of Information Technology Page 8
  • 9. Attack on Cryptography  Cryptanalysis plaintext break encrypt ‣ Ciphertext-only attack encrypt Key key ciphertext key ‣ Known-plaintext attack plaintext key key ‣ Chosen-plaintext attack Faculty of Information Technology plaintext ciphertext ciphertext Page 9
  • 10. Asymmetric Key Cryptography (Public Key Cryptography) Faculty of Information Technology Page 10
  • 11. Asymmetric Key Cryptography  encrypt ‣ ‣ ‣ symmetric key cryptography key decrypt key Key Key key ‣ ciphertext Faculty of Information Technology Symmetric Key Encryption Page 11
  • 12. Asymmetric Key Cryptography  cryptosystem cryptography ‣ Public Key asymmetric key key key ‣ Private Key key Encryption  ‣ public key encrypt Faculty of Information Technology public key Page 12
  • 13. Asymmetric Key Cryptography Faculty of Information Technology Page 13
  • 14. Asymmetric Encryption  Public-Key Cryptosystem ‣ encrypt ciphertext decrypt plaintext P = Plaintext E(P, PKreceiver) = C ‣ public key private key encryption decryption ‣ Plaintext Faculty of Information Technology E(C, SKreceiver) = P C = Cipherte PK = Public K SK = Private public key private Public Key key Private Key Ciphertext Encryp Decryp tion tion Plaintext Page 14
  • 15. Public Key Cryptography  Public Key Cryptography one-way function ‣ One-Way Function (Multiplication) (Factorization) ‣ • x12 = 144 • x12 144 = 12x12 = 144x1 = 24x6, … Faculty of Information Technology Page 15
  • 16.  Symmetric Key Cryptography Asymmetric Key Cryptography ‣ Security Goals Confidentiality, Integrity, Authenticity, Av ailability Non-Repudiation ‣ Faculty of Information Technology Page 16
  • 17.  RSA Algorithm  Diffie-Hellman Algorithm ( Key Exchange)  Elliptic Curve Cryptography  Digital Signature Algorithm Faculty of Information Technology Page 17
  • 18. RSA  RSA Adi Shamir) Len Adleman) MIT Faculty of Information Technology Ron Rivest) RSA Page 18
  • 19. RSA  (prime number) p q ‣    gcd ‣ Faculty of Information Technology n = pq m = (p-1)(q-1) e 1<e<m e m e e m gcd(e, m) Page 19
  • 20. RSA Encryption  M M<n Public Key (e, n) ‣ Ciphertext C = Me mod n RSA Decryption  ciphertext C (d, n) ‣ Message Private Key M = Cd mod n  ‣ p = 5, q = 7, n = 35, m = 24 Faculty of Information Technology Page 20
  • 21.  n = 35, e = 5 Plaintext M Me Ciphertext = Me mod n L 12 248832 17 O 15 759375 15 V 22 5153632 22 E 5 3125 10 Faculty of Information Technology Page 21
  • 22.  n = 35, d = 29 Cd Ciphertext M= Cd mod n Plaintext 17 48196857210675091509141182522307000 12 L 15 12783403948858939111232757568359400 15 O 22 8.5164331908653770195619449972111e+38 22 V 5 E 10 Faculty of Information Technology Page 22