CS2107 Assignment 1 Solution

Introduction

This assignment takes the form of an information security capture-the-flag (CTF) style competition. In a CTF, participants solve problems involving security weaknesses to bypass defences to obtain a sensitive piece of information called the "flag" .
Acknowledgements

AY21/22), Weiu Cheng (AY22/23, AY21/22), Wen Junhua (AY22/23, AY20/21), Shawn Chew (AY 21/22), Chan Jian Hao
(AY21/22), Ye Guoquan (AY21/22), Debbie Tan (AY20/21), Jaryl Loh (AY20/21, AY21/22), Daniel Lim (AY20/21), Chenglong (AY19/20), Shi Rong (AY17/18, AY19/20), Glenice Tan (AY19/20, AY18/19), Ngo Wei Lin (AY19/20, AY18/19), Lee Yu Choy (AY20/21, AY19/20, AY18/19, AY17/18), Nikolas Tay (AY 16/17) and Jeremy Heng (AY 16/17).

This assignment is worth 15% of the grade for the entire module. Assignment 1 is divided into the following sections:
1. Easy (75 points): Answer all challenges.
2. Medium (60 points): Answer at least 3 challenges from the given 6 challenges to get 60 points; the points from answering medium-level challenges are capped at 60.
3. Hard (15 points): Answer at least one challenge; Solving the other challenge earns you 15 bonus points.
To illustrate how the point calculation is done, you can consider the following 2 examples. Suppose Bob correctly answers all easy challenges, 4 medium challenges, and 0 hard challenges. Bob obtains: 75+60+0=135. Alice, meanwhile, correctly answers all easy challenges, 2 medium challenges, and 2 hard challenges. Alice obtains: 75+40+30=145. Alice actually earns her 15 bonus points, which are then used to directly top up her A1 points.
Contact

Please direct any inquiries about the assignment to
1. kelzin@u.nus.edu (Tan Kel Zin)
2. weiucheng.tan@u.nus.edu (Tan Weiu Cheng) 3. wen_junhua@u.nus.edu (Wen Junhua)
4. c.akash@u.nus.edu (Akash Chandrasekaran)
5. dcssu@nus.edu.sg (Prof. Sufatrio*)
Note that the TAs will not be debugging your code, but will only be around to discuss high level ideas. Do allow 3 working days for replies. Discussion on forums are highly encouraged.
*: Please cc me if you email your queries about the given challenges; For issues with access to the CTFd server, please email your TAs.
Rules and Guidelines

PLEASE READ THE FOLLOWING BEFORE BEGINNING
1. You are required to log in to https://cs2107-ctfd-i.comp.nus.edu.sg/ (accessible only within NUS Network) to submit flags.
All source codes and scripts, if any, in their respective folder based on the challenge name.
3. Do not attack any infrastructure not explicitly authorised in this document.
4. Multiple flag submission is permitted on the scoring platform without any penalty, but no bruteforcing of flag submission on the server will be tolerated.
8. All challenges have a solution. They are guaranteed to be solvable with assistance of the internet and some research.
9. Ask the TAs for assistance only after you have exhausted every other avenue of self-help.
10. Every challenge will contain a flag and will provide the accepted flag format. Please ensure your submissions meet the flag format stated exactly. This means include the CS2107{} portion unless otherwise stated.
One of the most important skills in the information security field is the skill of seeking an answer independently. It is expected that the participant be able to utilise resources discovered through Google or any other search engine to achieve the tasks.
While the challenges might not be covered in entirety in class, the topics in the assignment are very applicable to security problems in real life. In the long run, the practical skills gained would benefit participants immensely.

Linux Environment

A Linux system is crucial for solving some of the challenges, the challenges in this section will prepare you for the more advanced sections by presenting some elementary tasks to solve. It is expected that the participant has rudimentary proficiency in using a Linux system that can be gleaned by reading the tutorial at this link: https://www.digitalocean.com/community/tutorials/an-introduction-to-the-linux-terminal.
However, more knowledge might be needed, and it is expected that the participant do some self-exploration.

The nc Command

Throughout the assignments, if you see challenge with nc aaa.bbb.ccc.ddd xxxx , then it means that the challenge is hosted on the aaa.bbb.ccc.ddd server on xxxx port.
You can connect to the server by using the nc command in your terminal. In short, you can just copy & paste nc aaa.bbb.ccc.ddd xxxx and run it directly.
If you wish to host a TCP server locally, you can use ncat ncat -lvk -p 15000 -e "python3 main.py" Then connect to it with nc localhost 15000
Python3 Cheatsheet

Some challenges in the assignment might require some scripting to solve. Although you can use any programming languages you prefer, we recommend Python3. This is because Python3 has many useful libraries (PyCryptodome) that can deal with Cryptography scheme.
Python3 differentiates string and bytes with the b'' syntax

Here are some of the useful commands that is supported natively in Python3:
s.encode() - Convert string s into bytes
b.decode() - Convert bytes b into string
b.hex() - Convert bytes b to hex string
bytes.fromhex("01abcd") - Convert the hex string 01abcd to bytes bytes([1,2,3]) - Convert integer list to bytes
list(b) - Convert bytes - Convert b into a list of integers i = 0x1235 - Set the value of i to be the value 0x1235 i = int("1234ab", 16) - Convert hex string "1234ab" to integer pow(c, e, m) - Calculate c^e mod m
For most of Cryptography library in python3, they require the plaintext to be bytes and not a string. This is because a string in python3 might have different encoding, but the encoding for bytes is universally UTF-8
Here are some of the useful commands that is supported in PyCryptodome:
Crypto.Util.number.long_to_bytes(m) - Convert integer m to bytes Crypto.Util.number.bytes_to_long(b) - Convert bytes b to integer
Crypto.Util.Padding.pad(b, x) - Pad bytes b so that the length is multiple of x
Crypto.Cipher.AES.new(key, AES.MODE_ECB) - A new AES instances in ECB mode
To dynamically with interact with TCP server, you can use pwntools

Note that all the received message are in bytes. So you might to some conversion if necessary.
You can also change to debug mode with r = remote("123.123.123.123", 15000, level='debug')

Answer all challenges.
E.1 Sanity Check (15 mark)
A flag, written in our flag format, is placed somewhere in the assignment instruction file.
Try to find and submit it!
Flag format: CS2107{...}
Author: Akash
Julius Caesar was a smart man, yet not smart enough since he shifts each letter in his message by the same amount. For me, I shift my message a little differently depending on each letter's position in the message.
This is what I do: I first generate a random number 0 <= k <=26 ; then the letter at the i-th index of the flag is shifted k+i times to the right (mod 26). Can you decrypt my ciphertext and tell me its plaintext?
Author: Weiu Cheng
My .jpg image is encrypted in AES-CBC , can you recover the image if I tell you the following? - Key: cs2107isveryfun! - IV: ThisIsJustAnIV!! Please tell me the secret flag as shown in the image!
Author: Junhua
Find the MAC of the given file using HMAC with MD5, and put the MAC as the flag. The key used for the MAC is: cs2107isTheBestModEver
Flag format: CS2107{mac_here}
Author: Junhua
RSA is one of the most important public-key encryption algorithms. Do you know how to use it? Let me give you my c, p, q and e values, and please tell me the message m!
Author: Weiu Cheng

One-Time Pad is a secure cipher. But I don't like it if the key contains a '' byte. This is since any byte XOR-ed with '' is just itself. Hence, in my usage of One-Time Pad, I make sure that my keys have no '', so you won't get to see ANY BYTE of the flag at all!!! I'm smart, right??
You can repeatedly connect to the server to get different ciphertexts of the same flag encrypted using One-Time Pad with a different key each time. For each encryption, I have used a strong random key (with the same length as the flag) from urandom, but with no '' bytes in the key. Show me that you're smarter than me by finding out the flag! Server: nc cs2107-ctfd-i.comp.nus.edu.sg 5053
Author: Weiu Cheng
The following ciphertext is from an encryption with AES-CTR mode:
7d294f3c7a71e2808e8ce5afd9eb99c343460ca9f5f89ff062fed96b (in hex).
But, I only used 8-digit number as the key, and the following IV: 0000000000000000 .
Now, I'm not sure if it is secure enough. Can you decrypt the ciphertext, and get the plaintext?
Author: Junhua
I dare to challenge you to find a collision for my hash algorithm!
Server: nc cs2107-ctfd-i.comp.nus.edu.sg 5052
Author: Kel Zin
I want to create a program for my tool. However, I do not want to do it myself. Luckily for me, I've found this organization that can do it for me FOR FREE, yay! :D.
Now, how would I go about making that free request.... Their company website is here.
I heard there is a higher chance of free software if we mention one of their employees.
To achieve my goal, maybe I should send them a phishing message on Telegram based on the information I can find online? Author: Junhua
I'm wondering why nobody is using AES-ECB. Since AES is secure, my intuition is that it should be pretty secure.
The server provides an encryption oracle for AES ECB. Hence, you can query plaintexts of your choice to the server to get the corresponding ciphertexts. Tell me if you can see the FLAG appended to your chosen plaintext. Server: nc cs2107-ctfd-i.comp.nus.edu.sg 5051
Author: Kel Zin
RSA again??? Since you already know how it works, there shouldn't be a problem solving this... right? Now, you can have my c, e, n and 7p-3q values. Please tell me the message m this time again!
Author: Weiu Cheng

RSA is so secure, My prof always encrypts his emails using RSA.
Author: Kel Zin
This criminal organisation doing sus... Help us gain access to their server.
Server: nc cs2107-ctfd-i.comp.nus.edu.sg 5054
Author: Kel Zin
CS2107{let_the_games_begin}