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# Cryptography - PowerPoint PPT Presentation

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.

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## PowerPoint Slideshow about 'Cryptography' - yori

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Presentation Transcript

• 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

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.

• Efficient

• Large number of keys required for large number of communicating parties.

Security

Figure 29.3Symmetric-key cryptography

• Substitution cipher

• Monoalphabetic

• Polyalphabetic

• Transpositional 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.

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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.

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Figure 29.7Transpositional cipher

Figure 29.8Block cipher

Figure 29.11Product block

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.

• The number of keys required for a large number of users is greatly reduced.

• 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

• To decrypt a message:

• P = Cd mod N

where C is the ciphertext value

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• 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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