# 1337UP LIVE CTF 2023

## Over The Edge

> Numbers are fun!! ?
>
> Author: kavigihan
>
> [`over_the_edge.py`](https://raw.githubusercontent.com/D13David/ctf-writeups/main/1337uplive/pwn/over_the_edge/over_the_edge.py)

Tags: _pwn_

## Solution
We get a small python script for this challenge. The interesting bit is in `process_input`.

```python
def process_input(input_value):
num1 = np.array([0], dtype=np.uint64)
num2 = np.array([0], dtype=np.uint64)
num2[0] = 0
a = input_value
if a < 0:
return "Exiting..."
num1[0] = (a + 65)
if (num2[0] - num1[0]) == 1337:
return 'You won!\n'
return 'Try again.\n'
```

This obviously is a buffer underflow. `num1` and `num2` are both `uint64` values, whereas `num2` always is `0`. We basically need to find a number that gives us `0 - (x+65) = 1337`. We can calculate this like `(1<<64)-1402` (as `1337+65 = 1402`), which gives us `18446744073709550214`. If we enter this as input the value will underflow and we will get the flag.

Flag `INTIGRITI{fUn_w1th_1nt3g3r_0v3rfl0w_11}`

Original writeup (https://github.com/D13David/ctf-writeups/blob/main/1337uplive/pwn/over_the_edge/README.md).