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(a) List any four characteristics that
makes a “good” encryption algorithm. [4]- The
set of keys and algorithm should be free from complexity
- The implementation of process should be as simple as possible
- Errors in ciphering should not propagate and cause corruption of
message
- The size of the enciphered text should be no larger than the plaintext
(1 mark each)
(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]
Ans: Bpub
(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]
Ans: Apriv
(iii) Can A achieve both these goals simultaneously? If so, how, if
not, why
not? [1]
Ans: Yes, both goals can be achieved simultaneously,
by encrypting
with both keys, one after the other. (No explanation, no mark).
(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
2 marks for completely correct plaintext
(only 1 mark for partially correct plaintext). The remaining 3 marks
for working; the key is the grid:
aauan
dltme
iceat
gorch
imiha
tpsit
|
cvlre
aeepb
npmey
srsoc
oofpa
lbolr
|
rurnn
ytust
iicgo
nntii
gsivt
otoex |
so give 3 marks
for this grid, 2 marks for some other rectangular grid,
and 1 mark for any other working that is not complete nonsense.
(d) Explain any three disadvantages
of symmetric key protocol. [3]
- If the key is revealed (stolen, guessed,
bought or otherwise), the
interceptors can decrypt all the information in both directions
(1 mark)
- Distribution of keys becomes a problem, as keys must be transmitted
with utmost security (1 mark)
- The number of keys increases with the square of the number of users
exchanging secret information (1 mark)
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