ASCII Hunt CTF Challenge Writeup

Challenge Information

Name: ASCII Hunt
Points: 1
Category: Crypto
Objective: Decode the flag from a given set of arrays using missing numbers.

Solution

To solve the “ASCII Hunt” challenge, follow these steps:

Initial Analysis:
- You are provided with a large Python array consisting of multiple sub-arrays.
- Each sub-array has a length of 255, and there are 32 sub-arrays.
Understanding the Data:
- Each sub-array likely represents a set of ASCII values.
- Given that a flag is typically around 32 characters long, it is reasonable to assume that the 32 sub-arrays correspond to the characters in the flag.
Analyzing the Arrays:
- Went back and forth to the description and eventually found that each sub-array should ideally contain all values from 0 to 255.
- Identify that some values are missing from these arrays.

Finding Missing Values:

Write a script to find the missing values in each sub-array. This involves sorting the arrays and determining which numbers from 0 to 255 are absent.

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import ast

file_path = 'enc.txt'
with open(file_path, 'r') as file:
    file_contents = file.read()

arrays = ast.literal_eval(file_contents)
full_range = set(range(256))

flags = []

for array in arrays:
    array.sort()  # Sort the array
    # Find the missing numbers
    missing = list(full_range - set(array))
    missing.sort()  # Sort the missing numbers
    flags.append(missing[0])

print("Sorted Arrays: ", arrays)
print("Missing Numbers: ", flags)
print("".join([chr(int(char)) for char in flags]))

Constructing the Flag:
- The missing numbers in the arrays correspond to the ASCII values of the characters in the flag.
- Convert these missing numbers to their ASCII characters to reconstruct the flag.
Implementing the Script:
- Example Python script to find missing values:
Flag Retrieval:
- Run the script to find the missing numbers and convert them to characters.
- The resulting string is the flag.