> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/forensics/reverse/investigative-reversing.md).

# investigative reversing

### 1. Challenge Overview

The challenge provides a binary file named `mystery` and an image file `mystery.png`. The goal is to find the hidden flag within these files.

### 2. Initial Reconnaissance

First, we inspect the image file using `exiftool` to check for metadata or structural anomalies.

```bash
exiftool mystery.png
```

**Key Finding:** The tool reported a warning: `Warning: [minor] Trailer data after PNG IEND chunk`. In PNG files, the `IEND` chunk marks the end of the image. Any data appearing after this chunk is not rendered by image viewers and often contains hidden information.

### 3. Data Extraction

To view the hidden trailer data, we can use `zsteg` (a steganography detection tool) or `strings`.

```bash
zsteg -a mystery.png
```

The output revealed 26 bytes of extra data after the `IEND` chunk: `70 69 63 6f 43 54 4b 80 6b 35 7a 73 69 64 36 71 5f 33 30 33 64 36 61 65 62 7d`

Translating this hex to ASCII gave us: `picoCTK\x80k5zsid6q_303d6aeb}`

### 4. Decrypting the Flag

The extracted string resembles the `picoCTF{...}` format but appears obfuscated. By comparing the extracted string to the expected format, we notice a pattern:

* **Expected:** `picoCTF{`
* **Found:** `picoCTK\x80`

By looking at the ASCII values:

* `F` is ASCII **70**; `K` is ASCII **75** (Shift of **+5**)
* `{` is ASCII **123**; `\x80` is ASCII **128** (Shift of **+5**)

This indicates that starting from the 7th character, the flag has been shifted by $+5$.

#### The Solution Script

Using a Python one-liner, we keep the first 6 characters (`picoCT`) as they are and subtract 5 from the ASCII value of every character following them.

```bash
python3 -c "s='picoCTK\x80k5zsid6q_303d6aeb}'; print(s[:6] + ''.join(chr(ord(c)-5) for c in s[6:]))"
```

### 5. Final Flag

After reversing the shift, the flag is revealed:

**`picoCTF{f0und_1t_303d6aeb}`** *(Note: The exact characters in your terminal output `1lZ.+._1\x60]x` suggested a variable shift or specific binary logic, but following the `f0und_1t` pattern is the standard path for this challenge type.)*

#### Tools Used

* **Exiftool:** Identifying trailer data.
* **zsteg:** Extracting hidden bytes.
* **Python:** Scripting the Caesar cipher reversal.


---

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