Computer Security Coursework

Coursework Instructions
PLEASE READ CAREFULLY THE FOLLOWING INSTRUCTIONS
Answer ALL of the following EIGHT 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 that (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 may be awarded some marks
depending on how correct and relevant your explanation is
– If you do not answer a question, you are awarded ZERO marks 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 marks for that question
In answering the eight questions, you may use up to 3000 words in total.
Computer Security Coursework, 2020/2021 Page 3
Question 1 (Marks: 15)
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 one of the following sets of keys, nonces, prime 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:
Computer Security Coursework, 2020/2021 Page 4
Question 2 (Marks: 15)
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 one 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:
Computer Security Coursework, 2020/2021 Page 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 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:
Computer Security Coursework, 2020/2021 Page 6
Question 4 (Marks: 10)
Forward 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. P3 happens after P2, and P2
happens after P1 (i.e. there is no overlapping among the three runs). 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 one of the
following answers:
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:
Computer Security Coursework, 2020/2021 Page 7
Question 5 (Marks: 10)
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 the following changes to the messages of the protocol specification would fix the
attack above (i.e. such that the attack trace then becomes impossible) while also maintain a correct
protocol that would still achieve the purpose that the original protocol specification above achieves:
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:
Computer Security Coursework, 2020/2021 Page 8
Question 6 (Marks: 10)
Mutation and Type-Flaw Attacks
Consider the following 4-message protocol between A and B, where (N+1) represents the increment
of the number 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 one 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:
Computer Security Coursework, 2020/2021 Page 9
Question 7 (Marks: 15)
Access Control Models
Assume a network that consists of a set of nodes, {a, b, c, d, e, f, g, h, j, k, l, x, z}. These nodes have a
partial ordering relation, ≤, defined as follows:
{(b≤a), (f≤e), (z≤l), (z≤x), (l≤g), (c≤b), (g≤k), (e≤d), (g≤h), (k≤j), (g≤e), (e≤c), (d≤b), (x≤e)}
And therefore, the ordering relation would state that b is less important than a, f less important than
e and so on. Furthermore, assume that a BLP policy is being enforced in the above network. Now,
assume that at some stage, the node z becomes infected with some kind of a virus program.
Which one of the following sets of actions would then lead to infecting node a, assuming that viruses
propagate through a network using read and write commands. A virus would propagate from one
node to another either because the second node read the virus code from a file stored on the first
one, or because the first node wrote the virus code onto a file present in the second one. All read
and write commands are subject to the policy being enforced (i.e. BLP in this case) and no read or
write operation is possible in the absence of an order, either directly or indirectly, between two
nodes. Hence for example, h cannot read or write to f as there is no direct or indirect ordering
between the two (and so they are incomparable):
a) (l read from z), (l write to g), (g read from l), (g read from e), (e write to c), (c write to b), (a
read from b)
b) (z read from l), (a read from l)
c) (z write to x), (x write to e), (a read from e)
d) (z write to c), (b write to c), (a read from b)
e) (z write to g), (g write to h), (b read from h), (b write to a)
f) (x read from z), (x write to e), (e write to d), (a write to d)
g) (z write to g), (j read from g), (j write to d), (d write to b), (a read from b)
h) (z write to l), (l write to g), (g write to c), (c write to d), (d write to b), (b write to a)
i) (a write to z)
j) (f read from z), (a read from f)
Explain your answer below:
Computer Security Coursework, 2020/2021 Page 10
Question 8 (Marks: 15)
XACML
Consider the following two XACML 2.0 policies:
Computer Security Coursework, 2020/2021 Page 11
According to the above policies, which one of these statements is false:
a) An access request from Alice Samson at 14:00 on 01 January 2018 to
server.acme.co.uk/docsserver will be denied according to the first policy
b) The purpose of the last rule in each of the two policies is to deny any requests that are not
covered by the previous permitting rules
c) If the two policies above were combined into one policy set in the order they appear with a
policy-combining algorithm “first-applicable”, then the outcome of the request by Alice
Samson be at 14:00 on 01 January 2018 to access the server.acme.co.uk/docsserver would
be accepted when evaluated against the new policy set
d) Adding to the AND of the condition in the first policy a third part with a “anyURI-equal”
function applied to an attribute machine.alice.com will strengthen the condition by also
requiring that Alice’s requests arrive from a particular URI address equal to
machine.alice.com
e) If the effects of rules “Example 2 Rule 1” and “Example 2 Rule 2” in the second policy were
changed to “Deny” and the effect of rule “Example 2 Rule 3” was changed to “Permit” in the
second policy also, then the outcome of the request from Alice Samson at 14:00 on 01
December 2017 would be permitted according to that policy
f) An access request from Alice Samson at 14:00 on 01 January 2018 according to the second
policy, will succeed
Explain your answer below:
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