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(a) List any four characteristics that
makes a “good” encryption algorithm. [4]
(b) With a public key encryption, suppose A wants to send a message
to B. Let Apub and Apriv be A’s public
and private key, respectively; and similarly for B.
(i) If A wants to send a message to B in such a way that no-one other
than B
can received the message, what key should be used? [1]
(ii) If A wants to send a message to B in such a way that no-one other
than A
could have sent the message, what key should be used? [1]
(iii) Can A achieve both these goals simultaneously? If so, how, if
not, why
not? [1]
(c) Break the following columnar transposition cipher. The plaintext
is taken from a popular computer textbook, so “computer”
is a probable word. The ciphertext is broken up into blocks of five
characters for readability. [5]
aauan cvlre rurnn dltme aeepb ytust iceat npmey iicgo gorch srsoc
nntii imiha oofpa gsivt tpsit lbolr otoex
(d) Explain any three disadvantages
of symmetric key protocol. [3]
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