SENG6250 System and Network Security
School of Information and Physical Sciences
Semester 2, 2022
Assignment XXXXXXXXXXmarks, 10%) - Due: 21 August, 23:59
Aims
This assignment aims to establish a basic familiarity with security primitives and attacks by analysing,
demonstrating, designing solutions using cryptography.
Questions
1. Cryptanalysis on Monoalphabetic Cipher (20 marks)
A monoalphabetic substitution cipher generates the ciphertext below. Perform cryptanalysis
and find the plaintext. Note that the plaintext only includes meaningful English sentence(s).
Ciphertext
wep umpp rgmusfp
znj rwmpwfepk ngw wn s qsmyp powpzw agw sffnmkbzy wn ngm
s
gvcwbnz wep vswpmbsqr grpk smp cpmupfwqt rwmpwfesaqp.
Ciphertext letter frequency
a. Find the plaintext XXXXXXXXXXmarks)
. Show your process of finding (at least) FIVE plaintext letters. (15 marks)
2. Block Cipher and Operation Modes (15 marks)
Use an AES encryption calculator (e.g., https:
www.hanewin.net/encrypt/aes/aes-test.htm)
to demonstrate the Counter mode (CTR) with AES (CTR-AES).
a. Create a 256-bit key and a 512-bit plaintext (all in hexadecimal XXXXXXXXXXmarks)
The key should start with your student ID. For example, if your student ID is C1234567,
then your key can be:
C1234567EDEEEFF0F2F3F4F5F7F8F9FAC1234567EDEEEFF0F2F3F4F5F7F8F9FA
https:
www.hanewin.net/encrypt/aes/aes-test.htm
. Specify a hexdecimal Initialisation Vector (IV). An IV cannot be a trivial string like all 0s
or 1s XXXXXXXXXXmarks)
c. Demonstrate the process of each round in the CTR-AES. You can use the AES
encryption calculator to show the block cipher encryption result without providing
the encryption detail XXXXXXXXXXmarks)
d. Show the entire ciphertext of 512 bits XXXXXXXXXXmarks)
e. Please use the following format for your answers XXXXXXXXXXmarks)
Sample Format
Entire Plaintext: XXXX…XXXX
Key: XXXX…XXXX
IV: XXXX…XXXX
Round 1:
Input of AES: XXXX…XXXX
Output of AES: XXXX…XXXX
Round 2:
Input of AES: XXXX…XXXX
Output of AES: XXXX…XXXX
…
Entire Ciphertext: XXXX…XXXX
3. Hash Functions (20 marks)
a. Let ℎ be a secure one-way hash function. Given a set {??1,??2, ??3,??4, ??5}, such that
??1 = ℎ(??); ??2 = ℎ(??1); ??3 = ℎ(??2); ??4 = ℎ(??3); ??5 = ℎ(??4).
Suppose ??3 is known, can we compute any of others in {??1,??2, ??4,??5}? If yes, show
how; otherwise, explain why XXXXXXXXXXmarks)
. Let (??,??) be an RSA public key, and (??, ??,??) be the co
esponding private key. The
public key (??,??) is known to everyone, but NO ONE knows the private key (??, ??, ??).
Consider a message ??,
1) If 0 < ?? < ??, can we use the RSA encryption algorithm as a one-way hash
function? Justify your answer XXXXXXXXXXmarks)
2) If ?? > ??, can we use the RSA encryption algorithm as a cryptographic hash
function? Justify your answer XXXXXXXXXXmarks)
4. ElGamal Cryptosystem (20 marks)
Alice has a public and private key pair (??????, ??????) for ElGamal signature. Suppose that Alice
signs a message for Bob every day. If Alice uses the same random number ?? in all signatures.
Analyse the good (6 marks) and bad (14 marks) parts of this signing method.
5. Encryptions and Digital Signatures (25 marks)
Alice has a public and private key pair (??????, ??????) for RSA cryptosystem. Bob also has his public
and private key pair (??????, ??????) for the RSA cryptosystem. Suppose Bob wants to send a
message ?? to Alice. There are the following two ways.
Case 1: Bob uses ?????? to sign ?? and gets a co
esponding RSA signature ????. Then, Bob uses
?????? to encrypt (??,????). The ciphertext on (??,????) is sent to Alice.
Case 2: Bob uses ?????? to encrypt ?? and obtains the co
esponding ciphertext ??. Then, Bob
uses ?????? to sign ?? and obtains the signature ???? . The ciphertext ?? and the signature ???? are
sent to Alice.
a. Comparing two methods, what would be an advantage(s) of Case 1? (10 marks)
. Create/Find a scenario where we should use Case 2 rather than Case 1. Justify your
answer XXXXXXXXXXmarks)
Submission
All assignments must be submitted via Canvas (Assessment tab for SENG XXXXXXXXXXIf you submit more
than once, then only the latest will be graded. Your submission should be a PDF file containing answers
to all questions.
The mark for an assessment item submitted after the designated time on the due date, without an
approved extension of time, will be reduced by 10% of the possible maximum mark for that
assessment item for each day or part day that the assessment item is late. Note: this applies equally
to week and weekend days.
Plagiarism
A plagiarised assignment will receive a ZERO mark (and be penalised according to the university rules).
Assignment XXXXXXXXXXmarks, 10%) - Due: 21 August, 23:59