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APC811S - APPLIED CRYPTOGRAPHY - 2ND OPP - JULY 2022 |
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n Am I BI A u n IVER s ITY
OF SCIEn CE Ano TECH n OLOGY
FACULTY OF COMPUTING AND INFORMATICS
DEPARTMENT OF COMPUTER SCIENCE
QUALIFICATION:
BACHELOR OF COMPUTER SCIENCE HONOURS (INFORMATION SECURITY)
QUALIFICATION CODE: 08BHIF
LEVEL: 8
COURSE: APPLIEDCRYPTOGRAPHY
COURSE CODE: APC811S
DATE: JULY2022
SESSION: 1
DURATION: 2 HOURS
MARKS: 60
SECOND OPPORTUNITY / SUPPLEMENTARY EXAMINATION QUESTION PAPER
EXAMINER(S)
DR MERCY CHITAURO
MODERATOR:
MS ESNA MANGUNDU
THIS QUESTION PAPER CONSISTS OF 4 PAGES
(Excluding this front page)
INSTRUCTIONS
1. Answer all questions.
2. Write all your answers in the answer booklet provided and number every question.
3. Please, ensure that your writing is legible, neat and presentable.
4. Marks/scores per question are given in square brackets [ ].
5. Calculators are permitted
6. Do not use or bring into the examination venue books, mobile devices and other material that may
provide you with unfair advantage. Should you be in possession of one right now, draw the
attention of the examination officer or invigilator.
7. All things that should not be marked, e.g. any "rough work", have to be crossed out unambiguously.
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1.
Cryptolocker is a malware released in September 2013, Cryptolocker spread through
email attachments and encrypted the user's files so that they couldn't access them. The
hackers then sent a decryption key in return for a sum of money, usually somewhere from a
few hundred pounds up to a couple of grand (Norton.com, 2017).
a.
Which information security property is breached when a user is not able to
access files that they are authorised to view and modify?
[1]
b.
Explain why the legitimate users are not able view or access their files. [2]
c.
If the hacker gives the users, the correct decryption key will the users be able to
access their files?
[1]
d.
If your answer in 'le' is yes explain how the users will be able to access their
files. If your answer is no explain why the users will not be able to access their files.
e.
Given that the users are able to accessthe encrypted files. Why would the users
still not be able to understand what is contained in their files?
[2]
f.
Suppose the users had already encrypted their files before the hackers
encrypted them. Would the users have been able to access their files after they had
been encrypted by the hackers? Explain your answer.
[3]
g.
Which security property/objective would the users have achieved against the
hackers given the description in (lf).
[1]
2.
Public key encryption algorithms are used to distribute public keys and private
(session) keys for symmetric encryption algorithms.
a.
Explain how a public key is distributed using public key encryption
algorithms.
[3]
b.
Explain how a session key is distributed using public key encryption
algorithms.
[3]
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3.
Given the RSAalgorithm shown in Figure 1
Key Gcncrn1ion
elect,,. 'I
C lcul:!lc ,. = ,, x q
I' : nd q both pnme ·I' ,. 'I
Calcula1c = ¢(11) (JI - I (q - I
altulatr Ii
Public key
rri\\'ale key
de mod ,j,(11)= I
KU= {.-.11)
KR={d.11)
Plaim .,1:
Ciphmcxt:
Enrryp1Jo11
M II
C=M'(mn<l11J
Ciphertcxt:
Plaint . 1:
De.:ryption
M= C'1rmod11)
Figure 1: RSAAlgorithm
a.
Using the RSAalgorithm depicted in Figure 1 and given two prime numbers 5
and 7. Generate the public key and the private key. Fore and d use the smallest
value of d and e possible.
[8]
b.
Paul Kocher, a cryptographic consultant, demonstrated that a hacker can
determine an RSA private key by keeping track of how long a computer takes to
decipher messages. State and explain three countermeasures that can be used to
circumvent this attack.
[6]
4.
a.
Explain how to get a public-key certificate
[3]
b.
Explain how Zenane can verify Adelina's public-key certificate.
[8]
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r· '
5.
a.
DESis an example of which type of encryption algorithm?
[1]
b.
DES decryption rule is as follows: Use the ciphertext as input to the DES
algorithm, but use the subkeys Kn in reverse order. That is, use K16on the first
iteration, K1son the second iteration, and so on until K1is used on the 16th and last
iteration.
i. Which key is used on the first iteration?
[1]
ii. Which key is used on the sixth iteration?
[1]
c.
Given: the hexadeximal plaintext: 0123456789ABCDEF.
i.
Convert it to binary
[2]
d.
The first step of DESenciphering plaintext is the initial permutation (IP}given by:
58 so 42 34 26 18 10 2
60 52 44 36 28 20 12 4
62 54 46 38 30 22 14 6
64 56 48 40 32 24 16 8
57 49 41 33 25 17 9 1
59 51 43 35 27 19 11 3
61 53 45 37 29 21 13 5
63 55 47 39 31 23 15 7
What is the result of applying IP on your plaintext you got in (Sc}?
[5]
e.
IP you got in (Sd} is then divided into two equal sizes block; left half (L) and right
half (R). What is the size of each half?
[1]
f.
Write down left half (Lo}.
[1]
g.
Write down right half (Ro).
[1]
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3
e) Backformation
f) Eponym
g) Conversion
[1]
[1]
[1]
Total Marks [7]
Question 3
Complete the following table. Re-draw the table in your examination booklet.
Word
Keys
Waiter
Completion
Poetic
Healthy
Considering
Agreeing
redness
rotation
Free morpheme [5 marks]
Key
Wait
Complete
Poet
Health
Consider
Agree
Red
Rotate
Bound morpheme [5 marks]
s
er
ion
ic
y
ing
ing
ness
ion
Question 4
Total marks [10]
For each word below, draw a morphological tree diagram. Please note a word may belong to
two parts of speech, thus an option is given (e.g. N/V)
a) anticlimaxes
[3.5]
b) confrontational
[5.5]
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.:
6.
Consider the condition imposed on one-time pad: One-time pad - requires the use of a one-
time pre-shared key the same size as, or longer than, the message being sent. Why can you
not brute- force the pre-shared key for subsequent communications?
[2]
a.
Select in the table below whether it is true that the key given would be suitable
to be used as a pre-shared key for the plaintext to be encrypted with one-time pad.
Draw the table in your answer booklet.
[3]
Plaintext size
4000
150
603
Pre-shared key size
4005
135
603
True/False
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