> 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-required-3.md).

# Some Assembly Required 3

Detailed walkthrough will be available after 24 hours on medium!

<https://medium.com/@alhamrizvi.in/list/web-exploitation-picoctf-hard-f1416428808a>

this challenge demonstrates how client-side JavaScript can act as a thin wrapper while hiding the real logic inside a WebAssembly binary. initial analysis of the JS revealed that user input was being passed into a WASM module, making it clear that reversing the binary was necessary.

upon decompiling the WASM, we identified an encoded byte array (the flag) and a 5-byte key. each input character was XORed with this key before comparison. the tricky part was that the key indexing was reversed (`key[4 - (i % 5)]`), which caused incorrect results during the first decoding attempt.

after correcting the logic and applying the proper XOR operation, we successfully recovered the flag.

#### Exploitation Code

```python
# encoded bytes from WASM (offset 1024)
enc = [
0x9d,0x6e,0x93,0xc8,0xb2,0xb9,0x41,0x8b,0x94,0x90,
0xdd,0x3e,0x94,0x97,0x90,0xdd,0x3f,0xc4,0xc2,0xc9,
0xdd,0x34,0xc2,0xc5,0x97,0xdb,0x31,0x93,0x92,0xc0,
0xda,0x36,0x93,0x93,0xc1,0xd9,0x3e,0x91,0xc1,0x97,
0x90
]

# key from WASM (offset 1067)
key = [0xf1, 0xa7, 0xf0, 0x07, 0xed]

flag = ""

for i in range(len(enc)):
    flag += chr(enc[i] ^ key[4 - (i % 5)])

print(flag)
```

***

#### Output

```
picoCTF{3a09d0a084e8032bf66c5171c4049aff}
```


---

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