> 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/binary-exploitation/vault-door-1.md).

# Vault-door-1

### Vault-door-1 — PicoCTF Walkthrough (Reverse Engineering)

We’re given a Java source file where the program asks for input like:

```
picoCTF{...}
```

It then strips the prefix and suffix:

```java
String input = userInput.substring("picoCTF{".length(), userInput.length()-1);
```

So only the inner 32-character string is checked.

***

### Core Logic

The real validation happens here:

```java
return password.length() == 32 &&
       password.charAt(0)  == 'd' &&
       password.charAt(1)  == '3' &&
       password.charAt(2)  == '5' &&
       ...
```

👉 Each character is checked at a specific index.

***

### Exploit Idea

Instead of guessing:

* Create an empty array of size 32
* Fill each index using `charAt(position) == value`
* Reconstruct the string

***

### Reconstructed Password

```
d35cr4mbl3_tH3_cH4r4cT3r5_29e8d8
```

***

### Final Flag

```
picoCTF{d35cr4mbl3_tH3_cH4r4cT3r5_29e8d8}
```


---

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