1. 24-Nov-15Security Cryptography Cryptography is the science and art of transforming messages to make them secure and immune to attacks. It involves plaintext, encryption algorithm, ciphertext and decryption algorithm. Two types of cryptography exist: Symmetric-key cryptography and Public-key cryptography

2. Figure 29.1 Cryptography components

3. Figure 29.2 Encryption and decryption

4. 24-Nov-15Security Symmetric-key Cryptography (SKC) In SKC, the same key is used by both communicating parties. The algorithm used for decryption is the inverse of the algorithm used for encryption. Advantages: Efficient Disadvantages: Large number of keys required for large number of communicating parties.

5. Figure 29.3 Symmetric-key cryptography

6. 24-Nov-15Security SKC (Continued…) Traditional ciphers Substitution cipher Monoalphabetic Polyalphabetic Transpositional cipher Block ciphers P-Box S-Box Product Block Data Encryption Standard (DES)

7. 24-Nov-15Security Traditional Cipher Data encryption is based on character. Monoalphabetic One character will be represented by only one other character. Polyalphabetic One character may be represented by more than just one other character depending on the encryption algorithm used.

8. Figure 29.4 Caesar cipher

9. Figure 29.5 Example of monoalphabetic substitution

10. Figure 29.6 Vigenere cipher

11. 24-Nov-15Security Block Cipher Data encryption is based on block of bits P-box Transposition at bit level S-box Combines substitution and transposition (P-box) Product block More complex combination of P- and S-box DES Complex algorithm applied by the US government for nonmilitary and nonclassified use.

12. Figure 29.7 Transpositional cipher

13. Figure 29.8 Block cipher

14. Figure 29.9 P-box

15. Figure 29.10 S-box

16. Figure 29.11 Product block

17. Figure 29.12 DES

18. Figure 29.13 General scheme of DES

19. Figure 29.14 Iteration block

20. 24-Nov-15Security Public-key Cryptography (PKC) Two different keys are used, private key and public key. Private key is known only by the receiver. Public key is used by the senders. Advantages: The number of keys required for a large number of users is greatly reduced. Disadvantages: Complex algorithm used. Sender authentication is required

21. Figure 29.20 Public-key cryptography

22. 24-Nov-15Security RSA The private key is a pair of numbers N and e. The public key is also a pair of numbers N and d. N are common to both parties. To encrypt a message: C = P e mod N where P is the plain text value To decrypt a message: P = C d mod N where C is the ciphertext value

23. Figure 29.21 RSA

24. 24-Nov-15Security RSA (Continued…) To ensure the success of its application: Choose two large prime numbers p and q. Compute N = p x q. Choose e (less than N) such that e and (p- 1)(q-1) are relatively prime (having no common factor other than 1) Choose d such that (e x d) mod [(p-1)(q-1)] is equal to 1