Rating: 1.0

Writeup URL: [GitHub](https://infosecstreams.github.io/csaw21/checker/)

# checker

Writeup by: [GoProSlowYo](https://github.com/GoProSlowYo) solved by [Perryman](https://github.com/ghost)

Team: [OnlyFeet](https://ctftime.org/team/144644)

Writeup URL: [GitHub](https://infosecstreams.github.io/csaw21/checker/)

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```text
What's up with all the zeros and ones? Where are my letters and numbers? (This is a reversing challenge.)
```

----

## Initial Research

This one is extremely straightforward and they literally tell us: `(This is a reversing challenge.)`.

For this challenge you can work backwards through the provided python code and function calls to end up with the flag ultimately.

## Decode then `encode`

The main function calls an `encode` function on the `flag`:

`encoded = encode(flag)`.

If we look at the encode function we see it's setting `d = 24` then it's calling `x = up(input)`, `x = right(x,d)`, `x = down(x)`, `x = left(x,d)` and then returns `x`.

```python
def encode(plain):
d = 24
x = up(plain)
x = right(x,d)
x = down(x)
x = left(x,d)
return x
```

We need to take the encoded string and run backwards through main and each function.

----

## Victory

Submit the flag and claim the points:

**flag{[email protected]_Up}**

Original writeup (https://infosecstreams.github.io/csaw21/checker/).