Dust to Dust — DawgCTF SP26

Description

File: chal.zip

The repository entry only exposes the archive, so the practical challenge is to reverse the included encoder and decode output.txt. The recovered data is not normal plaintext: it is a packed monochrome bitmap that must be reconstructed into an image before the flag becomes readable.

Summary

encoder.c shows that the challenge takes a binary image, packs every 2x3 pixel block into one printable ASCII character, separates compressed rows with }, and terminates the stream with ~. Reversing that packing reconstructs the original bitmap, and rendering it reveals a handwritten DawgCTF flag.

Analysis

1. Read the encoder instead of guessing the format

The key artifact is encoder.c. Its input validation immediately tells us what the original data looked like:

• input.txt must contain only 0 and 1
• every row length must be a multiple of 3
• all rows must have equal length
• the number of rows must be a multiple of 2

That is a strong sign that the encoder is treating the file as a binary image or bitmap, not as text.

2. Understand the 2x3 packing rule

Inside compressArray, the encoder reads pixels in this order:

buffer[0] = arr[l*2][w*3];
buffer[1] = arr[l*2][w*3 + 1];
buffer[2] = arr[l*2][w*3 + 2];
buffer[3] = arr[l*2 + 1][w*3];
buffer[4] = arr[l*2 + 1][w*3 + 1];
buffer[5] = arr[l*2 + 1][w*3 + 2];

So each encoded character represents a 2-row by 3-column block:

top row:    b0 b1 b2
bottom row: b3 b4 b5

The six bits are parsed as a binary integer and shifted into the printable ASCII range:

long bin = strtol(buffer, NULL, 2);
c = (char)(0b00100000 + bin);

So the mapping is:

encoded_char = chr(int(six_bits, 2) + 0x20)

3. Understand the row delimiters

writeArray writes each compressed row followed by 0x7d (}), then appends 0x7e (~) once at the end:

fprintf(output, "%s%c", arr[i], (char)0b01111101);
...
fprintf(output, "%c", (char)0b01111110);

That means the decoding procedure is deterministic:

1. remove the final ~
2. split the stream on }
3. for each character in a compressed row, compute ord(ch) - 32
4. convert that value back to a 6-bit binary string
5. use the first 3 bits for the top row chunk and the last 3 bits for the bottom row chunk
6. append those chunks to reconstruct the full bitmap

4. Rebuild and render the bitmap

Once the rows are reconstructed, the output is a large black-and-white image. Writing it back as plain bitmap text or as a PBM image makes the content visible immediately.

The recovered image contains handwritten text spelling the flag. A few letters are visually ambiguous in the raw handwriting, but the correct final flag is the user-confirmed one below.

Solution

The decoder can be implemented in a few lines of Python:

#!/usr/bin/env python3

from pathlib import Path


def decode(path="output.txt"):
    data = Path(path).read_text()
    assert data.endswith("~"), "missing stream terminator"

    rows = []
    for packed_row in data[:-1].split("}"):
        if not packed_row:
            continue

        top = []
        bottom = []
        for ch in packed_row:
            value = ord(ch) - 0x20
            bits = f"{value:06b}"
            top.append(bits[:3])
            bottom.append(bits[3:])

        rows.append("".join(top))
        rows.append("".join(bottom))

    return rows


def save_outputs(rows):
    Path("recovered_input.txt").write_text("\n".join(rows) + "\n")

    width = len(rows[0])
    height = len(rows)
    with open("recovered.pbm", "w") as f:
        f.write(f"P1\n{width} {height}\n")
        for row in rows:
            f.write(" ".join(row) + "\n")


if __name__ == "__main__":
    bitmap_rows = decode("output.txt")
    save_outputs(bitmap_rows)
    print(f"Recovered bitmap: {len(bitmap_rows)} rows x {len(bitmap_rows[0])} cols")
    print("Saved: recovered_input.txt, recovered.pbm")

Step-by-step

1. Open encoder.c and notice it reads input.txt as a binary bitmap.
2. Observe that compression operates on 2x3 pixel blocks, producing 6-bit values.
3. Notice the printable ASCII bias: each 6-bit value is stored as value + 0x20.
4. Notice the structural delimiters: } separates compressed rows and ~ marks end-of-stream.
5. Reverse the transform by subtracting 0x20, re-expanding to 6 bits, and splitting the bits back into top and bottom 3-pixel chunks.
6. Save the reconstructed rows and render them as a monochrome image.
7. Read the handwritten flag from the recovered bitmap.