Network Security

  • Combination of low-cost powerful computing and high-performance networks is a two-edged sword:

    • Many powerful new services and applications are enabled
    • But computer systems and networks become highly susceptible(敏感) to a wide variety of security threats
    • Openness vs Security
  • Network security involves countermeasures(对策) to protect computer systems from intruders(入侵者)
    • Firewalls, security protocols, security practices, etc.


  • Information transmitted over network can be observed and recorded by eavesdroppers (using a packet sniffer)
  • Information can be replayed(重播) in attempts to access server
  • Requirements: privacy, authentication(认证), non-repudiation(否认)

Client Imposter

  • client imposter(冒名顶替者)

  • Imposters attempt to gain unauthorized(未经授权的) access to server
    • Ex. bank account or database of personal records
    • For example, in IP spoofing(戏弄) imposter sends packets with false source IP address
  • Requirements: privacy, authentication

Server Imposter

  • An imposter impersonates(模拟) a legitimate(合法的) server to gain sensitive information from a client

    • E.g. bank account number and associated user password
  • Requirements: privacy, authentication, non-repudiation

Denial of Service (DoS) Attack

  • Attacker can flood a server with requests, overloading the server resources (er. TCP Three-way handshake)

    • Results in denial of service to legitimate clients
  • Distributed denial of service attack on a server involves coordinated attack from multiple (usually hijacked) computers
  • Requirement: availability


  • The attacker sends a repeated same packet, to every port on the target server over using a fake IP address.
  • The server will send back ack continunously, prevents other client sending syn.

Man-in-the-Middle Attack

  • An imposter manages to place itself as man in the middle

    • convincing the server that it is legitimate client
    • convincing legitimate client that it is legitimate server
    • gathering sensitive information and possibly hijacking(劫持) session
  • Requirements: integrity, authentication

Malicious Code

  • A client becomes infected with malicious code
  • Virus: code that when executed, inserts itself in other programs
  • Worms: code that installs copies of itself in other machines attached to a network
  • Requirements: privacy, integrity, availability

Security Requirements

Security threats motivate requirements:

  • Privacy: information should be readable only by intended recipient(接受者)
  • Integrity: recipient can confirm that a message has not been altered during transmission
  • Authentication: it is possible to verify that sender or receiver is who he claims to be
  • Non-repudiation*(不可抵赖性): sender cannot deny having sent a given message.
  • Availability: of information and services


  • Secure communication channels

    • Encryption
    • Cryptographic checksums and hashes (加密校验和和散列)
    • Authentication
    • Digital Signatures
  • Secure borders
    • Firewalls
    • Virus checking
    • Intrusion detection(入侵检测)
    • Authentication
    • Access Control (访问控制)

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