CTFlearn-Writeups/Programming/Dawn's Lawn

Dawn's Lawn

The main idea finding the flag is some scripting in Python.

Step-1:

After downloading dawn.txt & dawn2.txt from the cloud, I tried to see their contents.

The contents of dawn.txt were:

When Dawn mows the lawn, she always does it in columns. She stars going down in the first one, then up in the second, down in the third, and continues until the entire lawn is done. Each tile in the lawn can be found in one of seven states:
* : a flower
| : grass 4
/ : grass 3
- : grass 2
\ : grass 1
_ : fertile dirt
. : infertile dirt

If a tile ever becomes 'infertile dirt', it will never be able to grow again. The flower is the highest stage of growth, while infertile dirt is the lowest. When Dawn's lawnmower is on a tile, it goes down 2 growth stages. A flower becomes grass 3, grass 2 becomes fertile dirt, grass 1 and fertile dirt becomes infertile dirt. Once the grass on that tile is trimmed, it gains the ability to grow quickly. The growth speed is the side length of Dawn's lawn. For example, in a 3x3 lawn, the growth speed is 3. If Dawn mows a flower, it becomes grass 3. After she mows the next 3 tiles, the grass 3 tile grows to grass 4. It will keep growing provided that Dawn is not finished mowing the lawn.

Here is a simple example lawn is a 3x3 lawn which looks like this:
*-*
_|.
/\/

This is how the lawn develops for every tile that Dawn mows. The arrows represent the direction that Dawn is travelling in (for the next tile), starting on the first tile in the top-left:
1:↓      2:↓      3:→      4:↑      5:↑      6:→      7:↓      8:↓       9
/-*  >>  /-*  >>  /-*  >>  |_*  >>  |_*  >>  |_*  >>  *\/  >>  *\\  >>  *\\
_|.  >>  .|.  >>  .|.  >>  .|.  >>  .-.  >>  .-.  >>  .-.  >>  ./.  >>  ./.
/\/  >>  /\/  >>  \\/  >>  \\/  >>  \\/  >>  -./  >>  -./  >>  -./  >>  /./

Flowers: 1
Remember that the growth rate changes for bigger lawns!
[Additional Test Cases]
=====================
5x5 Lawn: 3 flowers
\|_*\
_|/._
-.||-
\_-_-
|--_.
=====================
8x8 Lawn: 17 flowers
._\-_\|.
\|..**./
/|\|/*/\
/-.|\/|.
|/--\\./
_\-._.\|
|__.\|._
._\/||.*
=====================
15x15 Lawn: 83 flowers
_\|\*\_|\\._.-\
|_\-//.|..-|||.
_\\|*..*_-/-*|-
\\\_/*..-.|/-*\
._\|//_|\||.-|.
*..||\.\|||*||-
//*____*\\_|\\-
_._/__\|./-.__/
||\//.|//__./_\
\*|.-_|..|---.-
//._\//*-\/|*|-
|*_/*_*--.|-*|*
._/_*_||._-/_-|
-_*\__\\|*\-//|
.|/.*-./..-\/*-

The contents of dawn2.txt were:

\|*/|_|.-_\\|.|_.-//
/-\_.--.|-_._\.-|/*-
\\-|..-*-/__*--/.\*-
_/\|.*.---___***_/\.
_-|.\\././_/|.-|_\//
_-*.\..**/|/**.\_./-
|-*|*.-_-////.|**-|-
*\|*_-|_\-|__\_.*.-|
.*///*.*/*\_-\..*-**
*/_\-\.//--/||\\/_|_
*.**/--/**///./\\/-|
/|.\-..*-./\..-|\.||
|\/\__|./*_-\|-/_*_\
.|///*-/\-/|*/*||*-*
.\.|\/.*/--*.|\--\/\
/_/|_|_---\_\_.***.-
|-.._.-*\|*_\/_|_\/*
.\.|\|//_-|.*-*|\*|*
_\/-|-_*\-\|-/-/-*.-
-_..\_\_*\\-*__..*/-

Step-2:

I wrote a simple Flag.py script to calculate our answer.

The content of Flag.py is as follows:

import numpy as np
import sys

with open(sys.argv[1]) as f:

    lines = f.read().split('\n')
dim = len(lines[0])

conv = {'.': 0,'_': 1,'\\': 2, '-': 3, '/': 4, '|': 5, '*': 6}
matrix = np.array([[conv[y] for y in x] for x in lines])
matrix -= 2  # Mow everything
grow = np.tile(np.arange(dim-1, -1, -1), (dim, 1))
grow[matrix <= 0] = 0  # Don't grow on infertile land
matrix += grow
print("Flag: " + str((matrix >= 6).sum()))

Step-3:

After running the script as python3 Flag.py dawn2.txt, we get following output:

Flag: 194

Step-4:

Finally the flag becomes: 194