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Cryptography

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- 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

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Figure 29.1Cryptography components

Figure 29.2Encryption and decryption

- 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.

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Figure 29.3Symmetric-key cryptography

- Traditional ciphers
- Substitution cipher
- Monoalphabetic
- Polyalphabetic

- Transpositional cipher

- Substitution cipher
- Block ciphers
- P-Box
- S-Box
- Product Block
- Data Encryption Standard (DES)

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- 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.

- Monoalphabetic

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Figure 29.4Caesar cipher

Figure 29.5Example of monoalphabetic substitution

Figure 29.6Vigenere 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.

- P-box

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Figure 29.7Transpositional cipher

Figure 29.8Block cipher

Figure 29.9P-box

Figure 29.10S-box

Figure 29.11Product block

Figure 29.12DES

Figure 29.13General scheme of DES

Figure 29.14Iteration block

- 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

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Figure 29.20Public-key cryptography

- 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 = Pe mod N
where P is the plain text value

- C = Pe mod N
- To decrypt a message:
- P = Cd mod N
where C is the ciphertext value

- P = Cd mod N

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Figure 29.21RSA

- 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

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