Monash University
Faculty of Information Technology FIT2093 Introduction to Cybersecurity
● There are two parts to this test: Part A (30 marks, multiple choice questions) and Part B (70 marks, short answer questions).
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● This In-Semester test is worth 10% of your final unit mark.
● Answer all questions in a separate document.
● For Part A, each MCQ is allocated 2 marks. For Part B, marks for each question are
indicated at the beginning of each question and sub-question.
● The duration of this test is 120 minutes (2 hours), which includes a reading time of 10
● NOTE: The actual In-Semester Test will have a similar format as this Sample Test, but
will be run as a Moodle quiz. The Part A & B questions in the actual In-Semester Test may cover any of the week 1 to week 5 (inclusive) lecture topics.
PART A (30 marks)
Choose the correct answer a)-d) for each of the following questions.
1. The principle that security mechanisms must be easy to use is called the principle of ______.
a) timeliness
b) clarification c) effectiveness d) weakest link
2. Of the following, the mode that has an error propagation effect is:
d) all the above have the same error propagation
3. ShiftRows is a basic component of AES which is used to achieve ___________.
a) Substitution
b) Diffusion
c) Confusion
d) All of the above
4. When an attacker performs a capture of a data unit and its subsequent retransmission to produce an unauthorized effect, which attack is he performing?
a) Disruption
c) Masquerade
d) Service denial
5. An attack that takes unauthorised control of a remote controlled aircraft (drone) is a realisation of _________.
a) an information disclosure threat b) a repudiation threat
c) an inference threat
d) an integrity threat
6. The Caesar cipher is
a) secure due to the use of a long key
b) secure against brute force key search attack
c) insecure due to the use of a short key
d) secure against statistical frequency analysis attack
7. Using public-key encryption, what is the number of private keys Alice needs to keep to receive private messages from 10 people?
a) 2 b) 10 c) 1 d) 45
8. What is the multiplicative inverse of 2 mod 21?
a) 11 b) 14 c) 15 d) 17
9. Given integers (a, b) and a prime p, computing an integer z such that b = az mod p is called the _________.
a) Modular Exponentiation Problem
b) Discrete Logarithm Problem
c) RSA Problem
d) Greatest Common Divisor Problem
10. The security of RSA public key encryption is based on the computational hardness of the __________.
a) Discrete Logarithm Problem
b) Integer Factorisation Problem
c) ElGamal Problem
d) AES Problem
11. Which of the following statements is false regarding the RSA algorithm?
a) The security of the algorithm relies on the difficulty of integer factorisation
b) The relation between public exponent e and private exponent d is e × d mod φ(n) = 1
c) The value of e is chosen such that GCD(e, φ(n)) = n
d) If the ciphertext in a confidential message is C then the plaintext M = Cd mod n
12. Given Alice’s public key, a document D1 and a corresponding valid RSA digital signature s on D1 by Alice, an attacker Marvin cannot use s as Alice’s digital signature on another document D2 because
a) s will most likely not verify as a valid signature by Alice for D2 b) s will most likely verify as a valid signature by Alice for D2
c) The given Alice’s signature s does not depend on D1
d) There does not exist a valid signature by Alice on D2
13. In the Basic RSA signature (with no message hashing), if Alice’s public key is (n,e)=(35,7) and Bob receives a claimed signature s=2 by Alice on message m=23, would Bob accept s as a valid signature by Alice on m?
a) No, because 27 mod 35 is not equal to 23 b) Yes, because 27 mod 35 = 23
c) Yes, because 72 mod 35 = 23
d) None of the above
14. In the basic Diffie-Hellman key exchange protocol with public parameters (g,p)=(3,7), if Alice’s public key is 5 and Bob’s private key is 4, what is the shared key between Alice and Bob?
a) 3 b) 2 c) 6 d) 5
15. Alice and Bob used a Message Authentication Code (MAC) to authenticate messages sent to each other using a shared private key K. Later, Alice showed a message M sent by Bob with its corresponding valid MAC authenticator code T using key K to Cathy, but Cathy refused to accept T as a proof of the message M coming from Bob. Cathy’s refusal is ________.
a) Valid, because T could have been forged by Alice on M.
b) Invalid, because T could not have been forged by Alice on M.
c) Valid, because T does not depend on the message M.
d) None of the above.
END OF PART A
PART B (70 Marks)
Write your answers for each of these questions.
Q1. < Basic Concepts> (16 Marks)
Alice runs an online shop business and uses a MyBank Phone Banking App on her mobile phone to make payments to her employees from her business account at MyBank. An attacker Marvin downloaded and analyzed the MyBank Phone Banking App and found that the App uses a secure RSA encryption algorithm to encrypt the user’s password to the Bank as part of the Internet Banking authentication process. However, Marvin found that due to the way the App software was written, the run-time for the App software taken to encrypt a password reveals information about the password being encrypted. To exploit this vulnerability, Marvin managed to intercept the Internet communications between Alice’s phone App and the MyBank server, and measure the App’s response time to the server’s messages, allowing Marvin to eventually discover Alice’s MyBank Banking App password. Subsequently, Marvin used Alice’s password in his own copy of the MyBank App to impersonate Alice to MyBank and make a fraudulent transaction transferring funds from Alice’s business account into Marvin’s account.
(a) (8 marks) Categorise the type of attacks (passive or active) performed in the above scenario and identify which security goals are violated in the performed attacks. Explain the reasons for your answers.
(b) (8 marks) Categorise what kind of vulnerability was exploited in the attack above (design level or implementation level) and your reasoning. Explain what security principle is related to the above scenario and your reasoning.
Q2.
Figure 1 below shows the round function of a simplified variant of the AES block cipher for each round. It involves Substitute Bytes, Shift Rows and Add Round Key:
● Substitute Bytes: use S-Box to perform byte-by-byte substitution of the block. S-Box substitutions are defined in Table 2 (see Reference material below), which is interpreted as follows: Each individual byte is divided into leftmost 4 bits and rightmost 4 bits. Leftmost 4 bits are used as the row value while the rightmost 4 bits are used as column values. {95} refers to row 9 and column 5, thus {95} is mapped to {2A}.
● Shift Rows: The 1st row is not changed. The 2nd row is circularly left shifted by 1 byte. The 3rd row is circularly left shifted by 2 bytes and the 4th row is circularly left shifted by 3 bytes.
● Add Round Key: XOR the output from Shift Rows.
A message is divided into the blocks of 128-bit each and the 128 bits are arranged in a 4×4 byte matrix. The input bits R1…R128 in the first round are specified in the Table 1, the round key bits K1…K128 are all ones, and 16 S-box is specified in Table 2, see Reference material at end of test).
(a) (9 marks) Write the 128-bit output of Substitute Bytes as a 4 x 4 byte matrix in HEX. Show your working.
(b) (9 marks) Write the 128-bit output of the Shift Rows as a 4 x 4 byte matrix in HEX. Hint: A hexadecimal digit can be represented directly by 4 binary bits (and vice versa):
Figure 1: Simplified AES round function
Table 1: Inputs
Table 2: S-Box in AES 128 bits
Q3.
(a) Sally came up with a new public-key encryption system called SalCrypt. In the SalCrypt system, the public key is pk = (n,b), where n is a large random number and b is a large random number in the interval {1,2,…n-1}. Given a message m (a number in the interval {1,2,…,n-1}), Alice can encrypt the message m using Sally’s public key (n,b) by computing the ciphertext integer c = m + b mod n.
(9 marks) Explain what is the main security requirement on the encryption algorithm of any public-key encryption system. Do you think this security requirement is satisfied for the SalCrypt public-key encryption system? Explain why or why not.
(b) (9 marks) Compute the value of 211 mod 9 using the square-and-multiply algorithm shown in lectures. Show your working.
Q4.
(a) (9 marks) Consider the basic RSA Public Key Encryption system explained in the lectures. Suppose Alice generates private key prime numbers p = 3 and q = 11 and her public exponent e = 5. Compute Alice’s public key (e,n) and Alice’s private key d such that e × d = 1 mod φ(n), where φ(n) = (p-1)×(q-1). If Bob encrypts a message m = 2 to Alice, compute the RSA ciphertext c = m^e mod n that Bob sends to Alice.
(b) (9 marks) Suppose now that Alice uses a large random prime p in her RSA private key generation algorithm. However, an attacker Marvin discovered that Alice’s encryption app has the following vulnerability: instead of generating the prime q at random value when performing the key generation, to save time in key generation, the app always uses a fixed large prime q that is hard-coded into the app code. By downloading a copy of the software app Alice uses, Marvin found this fixed large prime q. Explain how given Alice’s public key (e,n) and Marvin’s knowledge of q, Marvin can efficiently decrypt ciphertexts encrypted using Alice’s public key.
END OF TEST
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