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Cryptography and Network Security . Basics of computer and network S ecurity. Computer security : involves implementing measures to secure a single computer ( protecting the ressources stored on that computer).

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basics of computer and network s ecurity
Basics of computer and network Security
  • Computer security: involvesimplementingmeasures to secure a single computer (protecting the ressources stored on that computer).
  • Network security: involvesprotecting all the ressources on a network. We must consider not only the computers on the network but other network devices and data transmittedacross the network.
basics of computer and network s ecurity1
Basics of computer and network Security
  • Security attack: any action that comprises the security of information owned by an organization.
basics of computer and network security
Basics of computer and network Security
  • Network security: canprovide one of the five services: confidentiality, integrity, authentification, nonrepudiation.
  • Confidentiality(Privacy): the transmitted message must makesense to only the intendedreceiver. To the others, the message must begarbage. When a costumer communicateswithherbank, sheexpectsthather communication istotalyconfidential.
basics of computer and network security1
Basics of computer and network Security

2. Integrity: the data must arrive to the receiverexactly as theywere sent.

A request for transferring 100$ should not bechanged to a request for 1000$.

3. Authentification: the receiverneeds to be sure of the sender’sidentity.

4.Nonrepudiation: a sender must not be able to denysending a message thathe or she , in fact, didsend.

  • Definition
    • Cryptography, a word with Greek origins, means “secret writing”.
    • It refers to the science of transferring messages to make them secure and immune to attacks.
  • Components
    • Plaintext : is the original message before being transformed.
    • Ciphertext: is the message after transformation.
    • Cipher : is the encryption and decryption algorithm.
    • Key: is an number (or set of numbers) that the cipher, as an algorithm, operates on.
  • In symmetric-key cryptography, the same key is used by the sender (for encryption) and the receiver (for decryption).
  • The key is shared.
  • In asymmetric-key cryptography, one key is used by the sender (for encryption) and other key is used by receiver (for decryption).
  • The keys are different.
simple example of symmetric key encryption
Simple Example of Symmetric Key Encryption
  • Shift Cipher ( or Caesar cipher)
  • in this cipher, the encryption algorithm is “shift key characters down”, key is N number
  • The decryption algorithm is “shift key characters up”, key is N number
  • Example:
    • Use the shift cipher with key=15 to encrypt the message “HELLO”? Show the ciphertext.
hash algorithm
Hash Algorithm
  • As the Cryptography is used to assure the privacy (confidentiality), Hash algorithm is used to ensure the integrity of the message.
  • At sender site, hash code is generated.
  • At receiver site, a new hash code for message is generated and compares with the hash code that sent, if match, message hasn’t been altered.
simple example of hash function
Simple Example of Hash Function
  • The hash algorithm: take every third letter of the message (ignore punctuation), and convert the letter to a number (a=1, b=2…z=26). Add the numbers together.

Hello, This is a sample message to demonstrate signatures.

12 +20 +19 +1 +13 +5 +19 +7 +15 +13 +19 +1 +19 +14 +21 +19

= 217 (therefore the hash value is 217)

digital signautre
Digital Signautre
  • Digital signature is a data string which associates a message (in digital form) with some originating entity.
  • Digital signature verification is a method for verifying that a digital signature is authentic (was indeed created by the specific entity).
  • The main difference from a handwritten signature is that digital signature of a message is intimately connected with the message, and for different messages is different, whereas the handwritten signature is adjoined to the message and always looks the same.