Network Security

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Network Security
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Security Threats

• Intercept
• Interrupt
• Modification
• Fabrication

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Security Threats

• Passive attacks
• Eavesdropping on, or monitoring, transmissions
• Electronic mail, file transfers, and
  client/server exchanges are examples of
  transmissions that can be monitored
• Active attacks
• Modification of transmitted data
• Attempts to gain unauthorized access to computer
  systems

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Encryption Methods

• The essential technology underlying virtually all
  automated network and computer security
  applications is cryptography
• Two fundamental approaches are in use
• conventional encryption, also known as symmetric
  encryption
• public-key encryption, also known as asymmetric
  encryption

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Conventional Encryption

• The only form of encryption prior to late 1970s
• Five components to the algorithm
• Plaintext The original message or data
• Encryption algorithm Performs various
  substitutions and transformations on the
  plaintext.
• Secret key Input to the encryption algorithm.
  Substitutions and transformations performed
  depend on this key
• Ciphertext Scrambled message produced as output.
  depends on the plaintext and the secret key
• Decryption algorithm Encryption algorithm run in
  reverse. Uses ciphertext and the secret key to
  produce the original plaintext.

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Conventional Encryption Operation
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Conventional Encryption Requirements Weaknesses

• Requirements
• A strong encryption algorithm
• Secure process for sender receiver to obtain
  secret keys
• Methods of Attack
• Cryptanalysis
• Brute force

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Public-Key Encryption

• Based on mathematical functions rather than on
  simple operations on bit patterns
• Asymmetric, involving the use of two separate
  keys
• Misconceptions about public key encryption
• it is more secure from cryptanalysis
• it is a general-purpose technique that has made
  conventional encryption obsolete

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Public-Key Encryption Components

• Plaintext
• Encryption algorithm
• Public key
• Private key
• Ciphertext
• Decryption algorithm

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Public-Key Encryption Operation
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Public-Key Signature Operation
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Characteristics of Public-Key

• Infeasible to determine the decryption key given
  knowledge of the cryptographic algorithm and the
  encryption key.
• Either of the two related keys can be used for
  encryption, with the other used for decryption.
• Slow, but provides tremendous flexibility to
  perform a number of security-related functions
• Most widely used algorithm is RSA

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Location of Encryption Devices

• Link encryption
• Each vulnerable communications link is equipped
  on both ends with an encryption device.
• All traffic over all communications links is
  secured.
• Vulnerable at each switch
• End-to-end encryption
• the encryption process is carried out at the two
  end systems.
• Encrypted data are transmitted unaltered across
  the network to the destination, which shares a
  key with the source to decrypt the data
• Packet headers cannot be secured

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Conventional EncryptionKey Distribution

• Both parties must have the secret key
• Key is changed frequently
• Requires either manual delivery of keys, or a
  third-party encrypted channel
• Most effective method is a Key Distribution
  Center (e.g. Kerberos)

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Public-Key EncryptionKey Distribution

• Parties create a pair of keys public key is
  broadly distributed, private key is not
• To reduce computational overhead, the following
  process is then used
• 1. Prepare a message.
• 2. Encrypt that message using conventional
  encryption with a one-time conventional session
  key.
• 3. Encrypt the session key using public-key
  encryption with recipients public key.
• 4. Attach the encrypted session key to the
  message and send it.

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Public Key Certificates

• 1. A public key is generated by the user and
  submitted to Agency X for certification.
• 2. X determines by some procedure, such as a
  face-to-face meeting, that this is authentically
  the users public key.
• 3. X appends a timestamp to the public key,
  generates the hash code of the result, and
  encrypts that result with Xs private key forming
  the signature.
• 4. The signature is attached to the public key.