April 2000
SC223 : COMPUTER SECURITY

QUESTION 2

Total Marks: 15 Marks

• Question 1
• Question 3
• Question 4
• Question 5
• Cover Page

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SUGGESTED SOLUTIONS
for Question 2

(a)Explain the differences with respect to key management between public-key and private-key cryptography.[4 marks ]

(b)Name and describe the problem with private-key cryptography that is solved by public-key cryptography.[2 marks ]

(c)Identify the components of a public-key cryptosystem,and explain the properties they must have.[6 marks ]

(d)One can use public-key encryption also for signing messages. However,doing so increases the possibility of insecurities.One well-known such insecurity arises if the recipient of a message encrypts and signs it and sends it back as conformation of receipt.
(i)Explain how time-stamps foil this attack.[2 marks ]
(ii)Describe another way of foiling the attack.[1 mark ]