> 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/web-exploitation/some-assembly-required2.md).

# some-assembly-required2

### 🔍 Challenge Overview

We are given a web challenge that validates input using **WebAssembly (WASM)** instead of plain JavaScript.

Unlike the previous challenge, the flag is **not directly visible via `strings`**, so we must reverse the logic.

### 🧩 Step 1: Extract WASM

From the website:

* Inspect JS → find `.wasm` file
* Download it:

```bash
curl -O http://wily-courier.picoctf.net:61986/<file>
```

Rename for convenience:

```bash
mv <long_name> OH
```

### 🧩 Step 2: Convert WASM → WAT

Install tool:

```bash
sudo dnf install wabt
```

Convert:

```bash
wasm2wat OH
```

### 🧠 Step 3: Identify Important Functions

From output:

* `copy_char` → processes input
* `strcmp` → compares buffers
* `check_flag` → main validation

***

### 🔥 Step 4: Understand Validation Logic

#### In `check_flag`:

```wat
i32.const 1072  ;; user input
i32.const 1024  ;; expected data
call strcmp
```

👉 So:

* Input stored at **1072**
* Expected flag stored at **1024**

***

### 🔥 Step 5: Extract Encoded Flag

At bottom:

```wat
(data (i32.const 1024) "xakgK\5cNsn08m?80jj:i;j:m0?klnm<i1:8>iljinu\00\00")
```

This is the **encoded flag**.

***

### 🔥 Step 6: Reverse Encoding

From `copy_char`:

```wat
i32.const 8
i32.xor
```

👉 Each character is transformed as:

```
encoded = input ^ 8
```

So reverse:

```
input = encoded ^ 8
```

***

### ⚠️ Common Mistake (YOU HIT THIS)

Your output:

```
picoCT=kF{...}
```

❌ Wrong because:

```
"\5c" ≠ literal "\" in Python string
```

👉 `\5c` = hex escape → **92 → ''**

So your encoded string must be written correctly.

***

### ✅ Correct Python Script

```python
encoded = b"xakgK\\5cNsn08m?80jj:i;j:m0?klnm<i1:8>iljinu"

flag = ""
i = 0

while i < len(encoded):
    if encoded[i] == 92:  # '\'
        # handle \5c manually
        hex_val = encoded[i+1:i+3]
        c = int(hex_val.decode(), 16)
        flag += chr(c ^ 8)
        i += 3
    else:
        flag += chr(encoded[i] ^ 8)
        i += 1

print(flag)
```

***

### 🏁 Final Flag

```
picoCTF{f80e708bb2a3b2e87cdfe4a9206adbaf}
```

***

### 💡 Key Takeaways

* WASM stores **raw data with escape sequences**
* Always watch for:
  * `\x??`
  * `\5c` → hex values
* XOR is reversible:

  ```
  A ^ B ^ B = A
  ```
* Client-side validation is always breakable


---

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