Binary Secrets - SMP CTF 2024 - Selection Round

The challenge provided two files:

1. Binary.txt - A file containing binary-encoded data.

2. Encode.py - A Python script that encodes text into binary based on a specific formula.

Analyzing the Encode.py File

The Encode.py script contains two functions:

• x(y, z, w):

  This function takes a string y, two integers z and w as input. Then the function multiplies the ASCII value of each character in the string y by a fixed number and converts the result into binary using bin(p)[2:].

  • The fixed number is (z + w) = 69 + 67 = 136.
  • Example: If the character is A (ASCII value 65), the script calculates 65 * 136 = 8840 and converts it to binary: 10001010001000.

• n():

  This function takes the flag, encodes it into binary using the function x(y, z, w), and prints the result.

Writing the Decoding Script

Using the reverse logic, I wrote a decoding script:

# Decoding the binary data
def decode(binary_data, z, w):
    decoded_text = ""
    for binary_value in binary_data.split():
        # Convert binary to decimal
        decimal_value = int(binary_value, 2)
        # Reverse the encoding formula to get the ASCII value
        ascii_value = decimal_value // (z + w)
        # Convert ASCII value to character
        decoded_text += chr(ascii_value)
    return decoded_text

# Input from Binary.txt
binary_data = """10110000011000 10100011101000 10101010000000 100000101011000 10010100110000
1101000001000 11001110001000 11011010111000 11001001111000 10010100110000 10100111111000
10101110010000 11001001111000 1001010011000 10100001100000 1100110000000 11111010110000
11010110101000 100001001101000"""

# Constants from Encode.py
z = 69
w = 67

# Decoding the binary data
flag = decode(binary_data, z, w)
print("Flag:", flag)

After running the script, it decoded the binary data and revealed the flag.