W4501 Capture the Flag I

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

This page will serve as a writeup for the CTF I competition problems.

Cryptography

ABC... It's as easy as 123...

[25 Points] Gabe passes secret notes to Gabby in class without the teacher noticing, but I managed to intercept one. The problem is, there are no words! What does this mean?
6-12-1-7 { 5-1-19-25 _ 1-19 _ 15-14-5 _ 20-23-15 _  20-8-18-5-5 }

This flag is encrypted using an A1Z26 cipher. An A1Z26 cipher simply converts each letter into the number of its position in the alphabet. For example, A is 1, B is 2, C is 3, and Z is 26. We can tell that this flag uses an A1Z26 cipher because no numbers exceed 26 and the challenge name references letters and their number counterparts. After putting the flag in an online A1Z26 decryptor we get that the flag is flag{easy_as_one_two_three}.

Ass Key

[25 Points] I stole this straight off the server’s hard drive, but it doesn’t make any sense. Maybe these numbers mean something? 0110011001101100011000010110011101111011010000010101001101000011010010010100100101011111011010010111001101011111010001010101101001111101
Hint: Computers encode legible text as binary numbers using a certain format.

This flag is the binary representation of ASCII characters. We know that it is in binary because only 0s and 1s are used, and it's likely encoded using ASCII because ASCII is the most common way to encode text. The challenge name and hint also imply that it uses ASCII. We can simply put this into an online binary to ASCII converter and we get that the flag is flag{ASCII_is_EZ}.

It's all about that base

[75 Points] The CS I students are getting really good at base conversions! But this number doesn’t look like any of the bases they’ve seen before… why don’t you convert it to decimal so the students understand it: 888888888
Hint: Look at the largest digit in the provided number.

Quantities can be represented in different ways called bases. We are used to base 10, which includes the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. However, there are many other bases. For example, computers use base 2, more commonly known as binary. Base 2 only includes the digits 0 and 1. We don't know the base that 888888888 is in, but the hint implies that it is in base 9 because base 9's largest digit is 8. When using an online tool to convert from base 9 to decimal (base 10), we get that the base 10 equivalent is 387420488.

Caesar Salad

[50 Points] My letters got jumbled 3 times! Unjumble them please: iodj{glg_Mxolxv_Pdnh_Wklv_Vdodg}
Hint: The names of these challenges help…

This flag is encrypted using a Caesar cipher. This cipher shifts letters in the alphabet by a certain amount. For instance, encoding the message “hello” with a shift of 3 results in “khoor.” We know that this flag is encrypted using a Caesar cipher because the flag appears to already use letters and the challenge name references Caesar. We know that the shift is 3 because of the challenge description, but we can also brute force every possible shift if necessary. When using an online converter, we get that the flag is flag{did_Julius_Make_This_Salad}.

Forensics

Extension Cord

[50 Points] My file isn’t working! Please fix it.
Hint: Computers need a way to tell different types of files apart.

The challenge also provides a file called extension_cord.txt. When we try to open extension_cord.txt, we get a lot of gibberish. This definitely is not intended to be a text file. If we change the file extension, the computer might then know what to do with it. We can start by changing the file extension to .png because PNG is a common file format and there are some hints of the PNG file header in the file's hex. When we change the file name to extension_cord.png and open it up we can see this image:

Flag from the "Extension Cord" CTF I challenge.

The flag is flag{dot_png_bro}.

Stalker

[25 Points] I want to find out where PewDiePie is hiding. I managed to get his IP address, will that help?
193.231.2.32
Hint: flag{USA_Allen}

This challenge is asking us to find a location using an IP Address. This can be done easily using an online tool. After putting the IP in an online tool that gets the location from an IP Address, we get that the country is Romania and the city is Bucharest. If we format the flag the way the hint says, we get that the flag is flag{Romania_Bucharest}.

Unzip... Unizip... Unzip... Unzip...

[175 Points] 1000 zips in a zip
1000 zips in a zip
Zip one out, check it out 
999 zips in a zip
Hint: Script the unzip

The challange also provides a file called 1000.zip. When examining 1000.zip, we find that it contains a file called 999.zip. If we continue to unzip these files, we find a pattern in which each zip file will contain another zip file with a name one number lower than the previous. Obviously, the goal is to unzip all the nested zip files until we reach 1.zip, which will likely contain the flag. Manually unzipping the files would take to long, so we should create a script. In this case, we made a Python script in Windows:

import os

count = 1000
while count > 0:
    os.system("7z e " + str(count) + ".zip")
    os.system("del " + str(count) + ".zip")
    count -= 1

This script uses the 7zip program to unzip a file, then deletes the old file to avoid clutter. When the script finishes execution, we find text file that contains flag{look_at_all_those_zips}.

	Coming Soon
	The rest of the challenges from CTF I will be added shortly.