> 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/reverse-engineering/b1ll_gat35.md).

# B1ll\_Gat35

I first tried debugging on a Windows 7(fuh because why tf you added a windows rev engineering cjallenge) setup using rpcclient, but the debugger(Ollydbg) failed to work, because i was fking using rpcclient and yes a dedass windows7 wasting on about an hour, I then moved to my Ghidra, where I was able to proceed successfully.

From Ghidra:

* Takes a number input
* Asks for a **key**
* Checks the key using a function
* If correct → prints flag
* If wrong → says incorrect

#### Important part:

```c
if (check_key(input) == 0)
    print("Incorrect")
else
    print("Correct") + show_flag()
```

So:

> If function returns **0 → wrong** If **non-zero → correct**

### Step 2: Easy Idea (Bypass)

Instead of finding the key, we **force the program to always go to "Correct"**

### Step 3: Patch the binary

In assembly:

```id="b2"
TEST EAX, EAX
JNZ correct_block
```

#### Meaning:

* `JNZ` → jump if NOT zero (correct case)

#### Trick

Change:

```id="b3"
JNZ → JNC
```

Why?

* `TEST` sets Carry Flag (CF) = 0
* `JNC` = Jump if CF = 0
* So it will **ALWAYS jump to correct block**

### Step 4: Run patched binary

```bash
wine patched.exe
```

Input anything:

```id="b5"
Correct input. Printing flag:
PICOCTF{These are the access codes to the vault: 1063340}
```


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