Binary Digits | CTF Writeups

Challenge

Category: Forensics · Points: 100 · CTF: picoCTF 2026

We are given a long string of binary digits and need to extract the flag from it.

Analysis

The file contains a raw binary string — a long sequence of 0s and 1s. The key insight is that every 8 bits maps to one byte, meaning the whole string is just binary-encoded file data.

Approach

Convert the binary string to bytes, write it out, and check what kind of file it is:

import sys

input_file = sys.argv[1]
output_file = sys.argv[2] if len(sys.argv) > 2 else "output.bin"

with open(input_file, 'r') as f:
    bits = f.read().strip().replace('\n', '').replace(' ', '')

data = bytearray(int(bits[i:i+8], 2) for i in range(0, len(bits), 8))

with open(output_file, 'wb') as f:
    f.write(data)

print(f"Done: {len(data)} bytes written to {output_file}")

Running this produces a .jpg image file. Opening it reveals the flag printed in the image.

Key Takeaway

Binary encoding isn’t encryption — it’s trivial to reverse. Any file can be represented as a binary string and decoded back in one line. Always check file headers (file output.bin) to identify the format before assuming it’s plain text.

Flag

picoCTF{h1dd3n_1n_th3_b1n4ry_2f96e9a1}