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Course Review. Examination Format. Two Sections: Section A: Compulsory (25%) : Generic areas Section B: Choose Three questions out of four questions (75%) Specialised areas such as: In-line encryptor, IPSEC, SSL/SET, Server side. Firewall, Policy NO Multiple Choice. Examination Content.

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Course review l.jpg

Course Review

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Examination Format

  • Two Sections:

  • Section A: Compulsory (25%) : Generic areas

  • Section B: Choose Three questions out of fourquestions (75%) Specialised areas such as:

    • In-line encryptor, IPSEC, SSL/SET, Server side. Firewall, Policy

  • NO Multiple Choice

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Examination Content

  • Test your

    • General awareness: definition

    • Understanding: description, calculation, drawing

    • In-depth knowledge : application

    • In-depth skill: problem solving

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Examination Hour

  • 2 hours: 120 minutes

  • Time management:

    • 100 minutes to work out your questions

    • 20 minutes: to review your work and read questions

    • Please note that 1 minute == 1 mark

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Examination Techniques

  • State or define: give the definition only, no need to say anything that is not relevant. For example, state the definition of security, no need to mention security is important to the Internet, pad lock is used to protect door etc.

  • Briefly describe or list: simply list out the points, no need to comment. For example, list two means to physically protect your server room. Install a pad lock and security alarm, no need to say that digital lock is better etc.

  • Compare: write sown the similarities and differences. For example, compare the pad lock and digital lock, you should write down the similarity : to protect the system, the difference: the former is to use a key while the latter is to use a password etc.

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Examination Techniques

  • Draw/sketch: draw a block diagram.

  • Suggest/give a solution: write down solution together with at least one sentence to explain why it works.

  • Determine/calculate: compute the values.

  • Describe: put down your detailed explanation.

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Suggestion for your review

1) Go through each lecture

2) Work out the on-line questions

3) Go through the materials from the textbook or additional information on the web

4) repeat 1 to 3, unless you finish all lectures

5) review the examination techniques

6) attempt the examination specimen to see whether you know how to do it.

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What is Computer Security? – L1

Computer security is a protection that is afforded to an information system in order to attain the applicable objectives or preserving the integrity, availabilityandconfidentiality of information system resources. The information resources include hardware, software, information and data.

Computer security

Three objectives

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Three layers on Security

  • Physical security

  • Layer 2 – in line encryptor, point to point

  • Layer 3 – IPSEC

  • Layer 4 – SSL/SET

  • PKI, symmetric key

  • Server

  • Firewall

  • Contents and Language, Java


Procedure or Policy


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Areas of Vulnerability

There are four basic items

  • Hardware: physical devices

  • Software: without the OS, applications and network, it cannot run

  • Data: the essence of computer systems

  • People: can cause a great deal of damage

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People – Computer Criminals

Four areas of computer crime

  • Theft of computer time: common practice to remote log into the system (not common in the Internet). This includes the time it takes to repair the computer system after infected by virus, bomb etc.

  • Theft of data: physical remove data from files

  • Manipulation of computer programs: change or insert/delete program

  • Software piracy: illegal copying of software

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Threats to security

  • Natural disasters: such as fire, floods, windstorms, earthquakes etc. We can do little to prevent natural disasters

  • Malfunctions: It cause much less damage, but occur frequently such as power surges, stray electrical forces, dust, operation error etc.

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Security Measures

  • Passwords: the most common means of user authentication. Generally used. Rules of choosing password:

  • Encryption: Encrypt the data. There are many standards such as Data Encryption Standard (DES) by IBM

  • Control: from planning to final implementation. This involves the progress review and acceptance test, post-installation review and periodic audits

  • Contingency planning: It is the backup plan in case an event my or may not occur. For example, if the application cannot operate, what should you do? (go back to manual system)

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Physical Protection – L2

Identify the natural disasters that threaten systems

Determine the damage assessment and reconstruction techniques

Design and select the physical location of a computer servers

Measure the air conditioning and power supply sources for computer center, servers and communication equipment

Describe the various access control mechanisms

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Type of Natural Disasters in Hong Kong





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Computer Room

  • A typical computer room with protection

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Web Security – L3

From ordinary users: it means the ability to browse the web in peace

For advanced users: it means the ability to conduct commercial transaction safely. For example, you are buying a toy over the Internet and is entering your visa number. You don’t want this information to be tapped by unauthorized persons.

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Three Parts of Web security


  • Web browser

  • The internet

  • Server




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Web security – three parts

  • Client-side security – To protect users’ privacy and integrity of his/her computer (browser)

  • Server-side security – To protect the server from break-ins and denial-of-service (sends huge garbage to make it unavailable)

  • Document confidentiality – To protect private information from being disclosed to third parties. .

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Risk to the Web server – three types

  • Webjacking – The website is redirected to other location or the content is modified. The term is similar hi-jacking.

  • Server break-ins – The server is broken by intruders. You can use firewall to protect your server.

  • Denial-of-service (Dos) attack – A hacker cannot break your site but can send a huge garbage to make your site unavailable.

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What is Cryptography? (L3)

  • The word comes from the Greek. It means “secret writing”.

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Four basic parts



How are you?






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How to Crack? (means break) (1)

  • Cryptanalysis: try to analyse the ciphertext to guess the meaning. For example, if the plaintext” How are you?” is converted to “uyi89rty89qwe89=“, we understand that “89” is used to replace the “space”. Of course, most of them are complicate and cannot be analyzed in this way.

  • Brute-force attack:Guess the key

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How to Crack?

  • If the key is two-digit number from 00 to 99, we can try 100 times, then we know the key. The average attempt is 100/2 = 50 times, as the key might be 12 or 86, we don’t need to try all of them. (In this case, the lock is an algorithm.)

  • An example, a numerical lock consists of three digits and you take 3 seconds to attempt one combination. How long you think you can break this lock? 3 x 1000/2 = 1500s = 25 minutes

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Types of Cryptography

  • Symmetric: use the same key to encrypt and decrypt the message

  • Asymmetric: sender uses recipients’ public key to encrypt and the recipient uses private key to decrypt.

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Asymmetric Cryptography

  • It is also called public key cryptography.

  • It uses two keys separately to encrypt and decrypt message which is safer than symmetric cryptography as the key cannot be reproduced.

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Digital Signature

  • The reversal of public key encryption

  • It uses sender’s private and public key rather than recipients’ public and private key.

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Example - encryption

  • Assume the plaintext is 1 1 0 0

  • And the key is 1 0 1 0

  • The encrypted message (ciphertext) using exclusive-or is:

    1 1 0 0 (plaintext)

    1 0 1 0 (key)

    0 1 1 0 (Ciphertext)

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Type of Ciphers

  • Stream cipher:

    • It is designed to accept a crypto key and a steam of plaintext to produce a stream of ciphertext

  • Block cipher

    • It is designed to take a block of a particular size, encrypt them with a key of a particular size and yield a block of cipertext block that is the same size of he plaintext block.

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Message Digest Functions

  • Message digest mean: Authentication without Encryption

    • Some times we only want authentication, but do not care about whether has been encrypted or not.

    • e.g., Message broadcast from authorized source (from Mr. Tung of SAR)

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Digital Envelope

Send both the ciphertext and digital envelope

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Certifying Authorities and the Public Key Infrastructure

  • The public key cryptography works well only the sender knows the recipient’s public key.

  • You can image the problem. If you have 100 recipients, you have to keep a small database of the 100’s public keys in your PC or ask the recipient to send you the public key.

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How you handle a signed certificate

  • Before the sender sends a secure message, the sender ask the recipients to present a signed certificate. (This has been certified by CA)

  • The sender decrypts the signed hash with the CA’s known public key to verify that the public key, name, and other identifying information.

  • The sender now uses the public key to send a message with confidence that it is the correct one.

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Link Encryption (L5)

What is Link Security?

Link security objectives by link encryption

In-line encryptor hardware

Point to point deployment

IP-routed development

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What is link security protocol?

  • Designed to hide secrets

  • Development to protect data against forgery (false data).

  • Can simply fit into existing Internet applications.

  • In Data link layer (layer 2)

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How to solve this?

  • Each plaintext message must have an extra information such as message number.

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Example of Rewrite

  • Here, the encrypted message is modified via a switcher.

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Deployment – Point to point

  • This deployment uses a pair of trusted lines between a pair of hosts.

  • There is no need to connect to the Internet.

  • For example, you can apply for a leased line via Pacific Century Cyber Work (PCCW) between two computers (example from Central to Kowloon Tong) or use VPN

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Point to point – Connection

  • Each host’s data link is connected to the plaintext port of in-line encryptor. It is commonly used in military applications.


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Site Protection – Unsafe arrangement

  • The workstation out of physical protection is unsafe.

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IPSec (Security at the IP Layer) L6

  • Security Objectives and issues associated with IPSEC

  • Overview of Network-Layer IP security

  • Cryptographic checksums for message integrity protection

  • IPSEC encryption and authentication headers

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Security Objectives

  • Security at the IP layer, called IPSEC, is a set of general purpose protocols for Protecting the TCP/IP communications in the Internet.

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Example – message consists of data and Crypto checksum

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Example – the message has been modified

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Modifying a Message protected with a cryptographic checksum

  • What happens if the hacker modifies the data such that the plaintext checksum is the same.

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Replay Attacks

  • The TCP/IP protocols are not explicitly designed to identify and reject packets that are cleverly collected and replayed.

  • TCP/IP protocols are designed to operate correctly even if data packets are transmitted.

  • There is no replay protection.

    • It does not mean that replay attack is not a risk. It is difficult to differentiate the duplicated packet or malicious replay.

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IPSEC Key Management

  • There are four types:

    • Manual Keying

    • Simple Key Interchange Protocol

    • Inter Security Association and Key Management Protocol

    • Photuris

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  • Secure Sockets Layer (SSL) is the dominant protocol for encrypting general communications between browser and server

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SET (Secure Electronic Transactions)

  • It is a specialised protocol for safeguarding credit-card-based transaction.

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SSL Characteristics

  • It is a flexible, general-purpose encryption system.

  • SSL protocol operates at TCP/IP transport layer.

  • One layer below application

  • This gives SSL flexibility and protocol independence

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Credit Card and its relationship







Card issued bank

Merchant’s bank

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Overview – L9

  • Active Content (not passive document)

    • Java and JavaScript

    • Browser as a security hole

  • Web privacy

    • Server logs

    • Cookies

    • Advices for users and Webmasters

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Bad Program by design or accident

  • We have to distinguish the programs that are designed to inflict harm (virus) or those that are well developed but still contains bugs.

  • Purposefully bad programs are more harmful. Example is Chaos Computer clubs’ malicious Active X control

  • Sometimes, it is difficult to distinct between software that is bad by intention and bad by accidents.

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Proxy Server

  • The proxy system is a system that help internet user to cache up the content they visited.

  • It simply stores the content that the users have visit and if later there are any user request the same content, it will provide the stored content to them.

  • The user will then get the content without really go out to the network.

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Location of proxy server

  • The trend is to integrate packet filtering and proxy system in firewall technology

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Server Site Security – L10

Why are Web sites Vulnerable (easily attacked)

Common questions about web site security

Steps to create a secure web site

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Security Policy

If there is no security policy, you are not sure whether your site is secure.

It is a list of what is and is not permissible.

For example, in the lab, you are not allowed to install illegal software.

Note that a security system consists of:

Technology, Policy and Law

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Unix Web Servers

Hardening a Unix Web server (means make it more secure)

Configuring the Web server

Monitoring logs

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Hardening a Unix Web server

  • Unix is a multi-user system.

  • It supports over hundreds of users with different directories and environments.

  • A user cannot modify or read a particular resources.

  • Users are grouped. (user: group:system) right

  • It is a general purpose system and is insecure. That is why we need to harden a Unix.

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  • Unix is not a perfect OS, we need to harden this by: downloading the latest patch, disable unnecessary services, minimise the number of users, etc.

  • Configure the Web server: minimise the use of privileged user, limit DOS

  • Monitor the logs: Unix system logs, server log and error log

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Server Security Checklist (1)

  • Have you installed all security-related patches?

  • Have you disabled all unnecessary services?

  • Have you run a security scanner on your system?

  • Does the server do double duty as a user workstation?

  • Do the Web server’s file permissions reasonable?

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Server Security Checklist (2)

  • Is the Web server running as root?

  • Is the Web server running any unnecessary features?

  • Have you established the limit of users?

  • Do you monitor system and web pages logs?

  • Do you monitor the integrity of the host?

  • Do you backup your system?

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What is a firewall?

  • In a traditional LAN system, all workstations can access the Internet with a result of equal attack from the outside.

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The location of a firewall

  • All traffic must go through the proxy server which then decides to accept or reject the traffic.

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Two basic Firewall Systems

There are two basic implementations for firewalls.

  • Dual home gateway firewall, the gateway machine has two network interface cards each of them is connected to the LAN (inter network) and the Internet (Outer network)

  • Screened-host gateway uses a router to forward all the traffic from/to the outer and inner networks.

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Dual-home gateway firewall

  • By default, the two networks are isolated.

  • However, there is a need to communicate between the inner and outer networks through the specialised programs called proxy (or proxies,many programs)

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Screen-hosted gateway

In fact, there is no effective difference between dual-home and screen-host

  • A network router is used to control access to the inner network. The router restricts communication between the outer and inner networks.

  • It ensures that the packets from the Internet can reach the well secured proxy which then examines the data.

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Law and Policy

  • Policy procedure – to write something so that the users can follow

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Good Luck

Work Hard

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