December 1999
SC223 : COMPUTER SECURITY

QUESTION 1 (Compulsory)

Total Marks: 30 Marks

• Question 2
• Question 3
• Question 4
• Question 5
• Cover Page

SUGGESTED SOLUTIONS
Solutions and allocated marks are indicated in green.
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Threats to system security can threaten confidentiality, integrity and availability.
Threats to system security:

(i) Distinguish between these three categories of threat. [3 marks]
Confidentiality refers to those services required to protect information from unauthorized disclosure;

integrity refers to those services required to ensure that information is accurate, complete and authentic;

availability refers to keeping the system’s hardware and software working efficiently.

(1 mark each.)

(ii) Which category covers looking over the shoulder of the person at the terminal next to you and reading information on their screen? [1 mark]
Confidentiality.

(iii) Which category covers an attack which swamps a network with millions of copies of the same email message? [1 mark]
Availability.

(iv) Which category covers a program that masquerades as a login screen, collecting user passwords? [1 mark]
Confidentiality.

(v) Which category covers breaking in to a college administration computer and changing your academic record? [1 mark]
Integrity.

Trojan horses, viruses and worms are three means of attacking computers by software.

(i) Give an example of each means. [3 marks]
Examples:Trojan horses, viruses and worms

Trojan horse: for example, masquerading as a normal compiler, but producing bogus object code which performs arbitrary actions with the user’s authority;

Virus: for example, the Melissa virus, which spreads by sending itself as an attachment to email from Microsoft Outlook;

Worm: for example, Robert Morris’ Internet Worm, which brought the Internet to a standstill in 1988.
(1 mark each; of course, there are many reasonable examples.)

(ii) What distinguishes trojan horses from viruses and worms? [1 mark]
Trojan horses do not replicate, whereas viruses and worms do.

(iii) What distinguishes a virus from a worm?[1 mark]
A worm is a standalone program, whereas a virus is attached to another program. (Or:) A worm replicates by itself, whereas a virus requires its host program to execute in order to replicate.

The RSA public-key cryptosystem uses mod N for encryption, and mod N for decryption. Which of E, D and N should be made public and which kept secret, and why?
Components of RSA algorithm:
• E is used for encryption, so needs to be public;
• N is the modulus, and is also needed for encryption, so should also be public;
• D is used for decryption, so should be secret.
(1markeach;no explanation,no mark.)

 Explain the similarities between written and digital signatures.
Any three of the following:
Signature is not reusable: signature is a function of the document, so cannot be transferred to another document;

Signature is unforgeable: only the sender knows the private key (Study Guide says ‘unforgettable’ here, which makes little sense — but it should receive credit);

Signature is authentic: receiver verifies the message using the sender’s public key;

Signature cannot be repudiated: receiver does not need sender’s help or cooperation to verify signature;

Signature is unalterable: if the signature is altered, it can no longer be verified.

(1 mark each, to a maximum of 3 marks; no explanation, no mark.)

 Identify and give an example of three classes of means of proving one’s identity to an authentication system.
Means of proving identity: Something you know: eg, a password;

Something you have: eg, keys, tokens, badges and smartcards (any one will do);

Something you are: biometric characteristics, eg, a fingerprint or retinal pattern.

(1 mark each; no example, no mark.)

 What is meant by the term biometrics? State the most and the least secure biometric devices in use.
Biometrics is the use of unique physical, behavioural and morphological characteristics to provide positive personal identification (1 mark). The most secure biometric characteristic is the retina pattern (1 mark); the least secure is the signature(1 mark)

 What is the main objective of disaster recovery? Explain the different approaches used to achieve this objective.
Disaster recovery aims to minimize the impact of a disaster (1 mark),andtoenjoya speedy and accurate recovery to normal operation (1 mark). Two approaches:
Avoid it: protect computer equipment and data against all anticipated risks;

If you cannot avoid it, reduce it: try to control the damage, should a disaster happen.

(1 mark each;no explanation,no mark.)