Tutorial 2 – ICT 211
Question 1
(a)
Suppose you have a network of n users.
You need to protect the communication in the
network such that a nonuser cannot read the messages transmitted.
On the other hand,
every user can read all the messages.
How many keys do you need if you are using a
secretkey algorithm to encrypt the message?
(b)
Explain the following types of cryptanalytic attacks:
(i)
Ciphertext only
(ii)
Chosen plaintext
(iii)
Chosen ciphertext
Question 2
(a) Consider a symmetric key algorithm with a 11bit key. Suppose you are using this algorithm.
How many times do you have to try to obtain the key for the following cases?
(i) the best case
(ii) the worst case
(iii) the average case.
Explain your answers.
(b) Consider the two plaintextciphertext pairs (P1, C1) and (P2, C2). The ciphertext C1 is
obtained by encrypting the plaintext P1 using an algorithm with a 20 bits key K1. The C2
is obtained by encrypting P2 using another algorithm with a 40 bits key K2.
(i)
Suppose exhaustive search is used to find the keys. Which key is more likely
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 Spring '10
 Tan
 Cryptography, Suppose exhaustive search

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