write-ups-challenges-2024-2025/arno/SOLUTION.md

Difficulty

Easy

Category

Steganography

How To Solve

From the filename it should be clear that LSB is a hint, referring to the Least Significant Bits from which you have to extract the flag. Using Python:

import wave

wav = wave.open("lesfilLeSduBorddemer.wav", mode='rb')
frames = wav.getnframes()
bytes = bytearray(list(wav.readframes(frames)))

# Replace each frame_byte with only the value of the LSB
for i in range(len(bytes)):
    bytes[i] = bytes[i] & 1 # bitmask with 00000001

# Byte array to string
result = ""
for i in range(0, len(bytes), 8):
    byte_chunk = bytes[i:i+8]
    int_value = int("".join(map(str, byte_chunk)), 2)
    char_value = chr(int_value)
    result += char_value

# Remove the trailing '#' characters
decoded = result.split("#")[0]

print(decoded) # This will show the flag
wav.close()

Flag

IGCTF{Th1s_m3ss4ge_1s_h1dd3n_1n_4ud10}