1
School of Computing

Schedule and Deliverables

Notes and Advice
● The Extenuating Circumstances procedure is there to support you if you have had any
circumstances (problems) that have been serious or significant enough to prevent you from
attending, completing or submitting an assessment on time. If you complete an Extenuating
Circumstances Form (ECF) for this assessment, it is important that you use the correct module
code, item number and deadline (not the late deadline) given above.
● ASDAC are available to any students who disclose a disability or require additional support for
their academic studies with a good set of resources on the ASDAC moodle site
● The University takes any form of academic misconduct (such as plagiarism or cheating) seriously,
so please make sure your work is your own. Please ensure you adhere to our Code of Student
Behaviour and watch the video on Plagiarism.
● Any material included in your coursework should be fully cited and referenced in APA 7 format.
Detailed advice on referencing is available from the library, also see TECFAC 08 Plagiarism.
● Any material submitted that does not meet format or submission guidelines, or falls outside of the
submission deadline could be subject to a cap on your overall result or disqualification entirely.
● If you need additional assistance, you can ask your personal tutor, student engagement officer
[email protected], academic tutor [email protected] or your lecturers.
● If you are concerned about your mental well-being, please contact our Well-being service
2
Coursework-related Instructions
Please read the following instructions carefully.
Answer ALL of the following SIX questions. Each question carries its own mark and the total
coursework mark is 100. The mark for each question and your coursework is calculated as
follows:
– If you choose the correct option among the multiple choices for a question, you will be
awarded a mark “up to” the maximum of the mark allocated for that question. Your
awarded mark for the question will depend on the explanation you provide as to why you
made the (correct) choice. If you provide no explanation whatsoever, and you chose the
correct answer, you will be awarded ONE mark only for that question.
– If you choose any of the wrong answers in a question, you get ZERO for that question.
– If you do not answer a question, you will be awarded ZERO for that question.
– Always choose only one answer per question. If you choose more than one answer in a
single question, you will be awarded ZERO for that question.
Please then submit your report containing all your choices and justifications of those choices in
a single PDF file, no later than the deadline marked on your Moodle submission link. If you
have a valid ECF or you are submitting late, please submit your report via the link marked for
ECF and Late submissions by the deadline indicated there.
3
Question 1 (Marks: 20)
Cryptographic Data Objects
B has just received the following message, which represents a cryptographic data object:
{(
{(KPbB)KPrS mod KPbS}K1,
{|(NB, NA, {{({K2}KPbB, NS)}(G1)KPrA mod NA}K1, {|{({G3}(KPbA)KPrS mod KPbS,
G2)}K1|}KPrB)|}KPrA
)}KBS
The following explains various terms in this object and some of the abbreviations used:
• {M}K represents the encryption of some message/data M using the key K
• {|M|}K represents the digital signing of some message/data M using the key K
• NX represents a nonce (i.e. a fresh and possibly random number used once only) generated
by X
• KpbX represents the public part of the key pair presumably owned by X
• KprX represents the private part of the key pair presumably owned by X
• KAB represents a symmetric key shared between A and B
• K (or K1, K2, K3 etc.) represents some arbitrary key with no assumptions about its scope
• M represents some alphanumeric/textual message with no assumptions
• G1, G2, G3 etc. are prime numbers
which of the following sets of keys, nonces, numbers, and alphanumeric/textual messages “best”
represents B’s knowledge, after B applies any number of possible cryptographic operations to the
object above, and assuming that B already has access to key K1 and the public key of any agent:
a) KBS , G2 , KPrB
b) {(KPbB)KPrS mod KPbS , G2 , KBS , KPrB , {(KPbB)KPrS mod KPbS}K1, NA , NB
c) NA , NB
d) NA , NB , KBS , KPrB
e) {(KPbB)KPrS mod KPbS}K1 , {|(NB, NA, {{({K2}KPbB, NS)}(G1)KPrA mod NA}K1, {|{({G3}(KPbA)KPrS
mod KPbS, G2)}K1|}KPrB)|}KPrA , NA , NB , KBS , KPrB , {(KPbB)KPrS mod KPbS
f) G2 , NA , NB , G1 , KBS , KPrB
g) (KPbB)KPrS mod KPbS , NA , NB , G2 , KBS , KPrB
h) (KPbB)KPrS mod KPbS , (G1)KPrA mod NA , NA, NB , G2 , KBS , KPrB
i) (KPbB)KPrS mod KPbS , G3 , G2 , KBS , KPrB
j) (KPbB)KPrS mod KPbS , NA , NB , G2 , KBS , KPrB , G3 , (KPbA)KPrS mod KPbS
k) NB
Explain your answer below:
4
Question 2 (Marks: 20)
Authentication Protocols
Consider the following 4-message protocol:
1. A → S: (B, {(A, K1)}KpbS)
2. S → B: A
3. B → S: (A, {(B, K2)}KpbS)
4. S → A: (B, {K2}K1)
Which of the following statements is true, at the end of the protocol, and with regards to the
purpose of the protocol:
a) Both A and B establish a session key K2, and B is sure of A’s identity
b) Both A and B establish a session key K1, and B is sure of A’s identity
c) Both A and B establish a session key K1, and A is sure of B’s identity
d) Both A and B establish a session key K1, and both B and A are sure of each other’s identity
e) Both A and B establish a session key K2, and A is sure of B’s identity
f) Both A and B establish a session key K1
g) Both A and B establish a session key K2
h) Both A and B authenticate each other by knowing each other’s identities
i) A ends up knowing B’s identity
j) B ends up knowing A’s identity
k) None of the above
l) All of the above
Explain your answer below:
5
Question 3 (Marks: 10)
Non-Repudiation and Anonymity Protocols
For the Zhou-Gollman non-repudiation protocol discussed in the lecture on “Non-Repudiation
and Anonymity Protocols”, which one of the following statements is false:
a) At time point 4, both A and B can produce evidence to prove that they received K
b) At time point 2, both A and B can produce evidence to prove that they received a signed
message from the other party
c) At time point 0, S cannot prove anything
d) At time point 3, B cannot produce evidence to prove that A has access to key K
e) At time point 1, A can prove that B is alive
f) At time point 4, S can prove that A is alive
g) At time point 3, S can produce evidence that that A has access to key K
h) At time point 0, A is not alive
i) At time point 2, A can produce evidence to prove that B is alive
j) At time point 4, the protocol terminates
Explain your answer below:
6
Question 4 (Marks: 15)
Forwards Secrecy Protocols
Consider the following 4-message protocol:
1. A → S: (B, {(A, K1)}KpbS)
2. S → B: A
3. B → S: (A, {(B, K2)}KpbS)
4. S → A: (B, {K2}K1)
Assume three runs of the above protocol, that we call P1, P2 and P3. If after completion of run
P3, K1 is compromised, i.e. it is leaked to some external intruder, how would this impact the
forward secrecy property of K2 for all the three runs of the protocol P1, P2 and P3? Choose the
right answer:
a) Compromising K1 in P3 compromises every other key in all of the three runs of the protocol
b) The secrecy of P3.K2 is not compromised, and therefore P2.K2 and P1.K2 would remain
secret
c) Compromising K1 in P3 compromises P3.K2, and therefore, every other previous version of
K1 and K2 are also compromised
d) The secrecy of P3.K2 is compromised, but P2.K2 and P1.K2 would remain secret since K1 is
refreshed after each run, therefore P3.K1 is different from P2.K1 and is different from P1.K1
e) Even though K1 is compromised in P3, K2 is not compromised in any of the three runs
Explain your answer below:
7
Question 5 (Marks: 15)
Attacks on Security Protocols
Consider the following 4-message protocol:
1. A → S: (B, {(A, K1)}KpbS)
2. S → B: A
3. B → S: (A, {(B, K2)}KpbS)
4. S → A: (B, {K2}K1)
And the following attack trace:
1. I(A) → S: (B, {(A, K)}KpbS)
2. S → B: A
3. B → S: (A, {(B, K2)}KpbS)
4. S → I(A): (B, {K2}K)
Which one of these changes to the protocol messages would fix the attack trace above, such as
the attack then becomes impossible:
a) 3. B → S: (A, {(B, {K2}KpbA)}KpbS)
b) 4. S → A: (B, {K2, A}K1)
c) 2. S → B: {A}KpbB
d) 2. S → B: B
e) 3. B → S: (A, {(B, {K2}KprS)}KpbS)
f) 1. A → S: {(B, A, K1)}KpbS
g) 1. A → S: (A, {(B, K1)}KpbS)
h) 4. S → A: (B, {K1}K2)
i) 4. S → A: (A, B, {K2}K1)
j) 2. S → B: A, B
Explain your answer below:
8
Question 6 (Marks: 20)
Mutation and Type-Flaw Attacks
Consider the following 4-message protocol between A and B, where (N+1) represents the
increment of N:
1. A → B: (A, {NA}KAB)
2. B → A: {(NA+1, NB)}KAB
3. A → B: {NB+1}KAB
4. B → A: {(K’AB, NA)}KAB
Which of the following mutations to messages of the protocol above, would constitute a harmful
attack:
a) 1. A → B: (C, {NA}KAB)
b) 1. A → B: ({NA}KAB, A)
c) 4. B → A: {(KAB, NA)}KAB
d) 4. B → A: {(K’AB, NB+1)}KAB
e) 3. A → B: {NB+1}KpbB
f) 2. B → A: {(NA+1, NA)}KAB
Explain your answer below: