24-Nov-15Security Cryptography Cryptography is the science and art of transforming messages to make them secure and immune to attacks. It involves plaintext,

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Presentation transcript:

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

Figure 29.1 Cryptography components

Figure 29.2 Encryption and decryption

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.

Figure 29.3 Symmetric-key cryptography

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)

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.

Figure 29.4 Caesar cipher

Figure 29.5 Example of monoalphabetic substitution

Figure 29.6 Vigenere cipher

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.

Figure 29.7 Transpositional cipher

Figure 29.8 Block cipher

Figure 29.9 P-box

Figure S-box

Figure Product block

Figure DES

Figure General scheme of DES

Figure Iteration block

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

Figure Public-key cryptography

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

Figure RSA

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