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Topic outline

 

  • Time: 25 hours
    Level: Masters

 
 

Introduction

  • Introduction Resource
  • Communication networks are used to transfer valuable and confidential information for a variety of purposes. As a consequence, they attract the attention of people who intend to steal or misuse information,...
 

1 Terminology and abbreviations

  • 1.1 Terminology Resource
  • Throughout this unit I shall use the terms ‘vulnerability’, ‘threat’ and ‘attack’. It is worthwhile clarifying these terms before proceeding:
  • 1.2 Abbreviations Resource
  • The table below shows the abbreviations that are used throughout this unit, and their meanings.
 

2 Background to network security

 

3 Threats to communication networks

 

4 Principles of encryption

  • 4.1 An introduction to encryption and cryptography Resource
  • Section 3 has introduced you to the main threats to network security. Before I begin to examine the countermeasures to these threats I want to introduce briefly one of the fundamental building blocks...
  • 4.2 An overview of symmetric key systems Resource
  • We can think of symmetric key systems as sharing a single secret key between the two communicating entities – this key is used for both encryption and decryption. (In practice, the encryption and decryption...
  • 4.3 The components of a symmetric key system Resource
  • I shall now explain the components of a symmetric key system in more detail.
  • 4.4 Asymmetric key systems Resource
  • Asymmetric or public key systems are based on encryption techniques whereby data that has been encrypted by one key can be decrypted by a different, seemingly unrelated, key. One of the keys is...
  • 4.5 Vulnerability to attack Resource
  • All the symmetric and public key algorithms listed in Table 2 and Table 3share the fundamental property that their secrecy lies in the key and not in the algorithm. (This is generally known as Kerchoff's...
  • 4.6 Hybrid systems Resource
  • As you have seen from earlier sections, a major advantage of asymmetric key systems over symmetric key systems is that no exchange of a secret key is required between communicating entities. However, in...
 

5 Implementing encryption in networks

 

6 Integrity

  • 6.1 Encryption and integrity Resource
  • You should recall from Section 3.2 that integrity relates to assurance that there has been no unauthorised modification of a message and that the version received is the same as the version sent.
  • 6.2 Other ways of providing assurance of integrity Resource
  • Some other method of providing assurance of the integrity of a message is therefore needed – some kind of concise identity of the original message that can be checked against the received message to reveal...
 

7 Freshness

  • 7.1 Introduction Resource
  • A message replay attack was introduced briefly in Section 3.4. In this attack a message, or a portion of a message, is recorded and replayed at some later date. For example, an instruction to a bank to...
  • 7.2 Time stamps Resource
  • A digital time stamp is analogous to a conventional postmark on an envelope: it provides some check of when a message was sent. Returning to the example of Alice and Bob, Alice could add the time and date...
  • 7.3 Sequence numbers Resource
  • Sequence numbers are an alternative way of indicating freshness. If Alice is sending a stream of messages to Bob she can bind each one to a sequential serial number, and encryption will prevent...
  • 7.4 Nonces Resource
  • This third method of freshness indication uses an unpredictable value in a challenge–response sequence. The sequence of events is illustrated in Figure 11. Bob wants to communicate with Alice but she needs...
 

8 Authentication

 

9 Access control

  • 9.1 Introduction Resource
  • In this section I shall discuss two major approaches used to restrict access to networks – passwords and firewalls.
  • 9.2 Passwords Resource
  • I have introduced encryption keys in previous sections. A password can also be thought of as a type of key in as much as it enables the keyholder to gain access to a particular resource. In Section 2.3,...
  • 9.3 Firewalls – an overview Resource
  • Firewalls play an important role in restricting and controlling access to networks. A firewall is normally implemented within a router or gateway, and will monitor incoming and outgoing traffic at the...
  • 9.4 Packet-filtering router Resource
  • A packet-filtering router either blocks or passes packets presented to it according to a set of filtering rules. Figure 14 shows this arrangement.
  • 9.5 Application level gateways Resource
  • An application level gateway is implemented through a proxy server, which acts as an intermediary between a client and a server. A client application from within the protected network may request services...
  • 9.6 Circuit level gateways Resource
  • A circuit level gateway operates at the transport layer of the OSI or internet reference models and, as the name implies, implements circuit level filtering rather than packet level filtering. It checks...
  • 9.7 Examples of firewall implementation Resource
  • In practice, firewalls are likely to be combinations of the types that I have described. For example, a screened sub-network is commonly incorporated in a firewall scheme, as shown in Figure 17. In this...
 

10 Summary

 

References and Acknowledgements

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