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## PowerPoint Slideshow about ' Cryptography' - yori

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Presentation Transcript

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

Security

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.

Security

SKC (Continued…)

- Traditional ciphers
- Substitution cipher
- Monoalphabetic
- Polyalphabetic
- Transpositional cipher
- Block ciphers
- P-Box
- S-Box
- Product Block
- Data Encryption Standard (DES)

Security

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.

Security

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.

Security

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

Security

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 = Pe mod N

where P is the plain text value

- To decrypt a message:
- P = Cd mod N

where C is the ciphertext value

Security

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

Security

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