Punchagle - 100 (Algo)
Write-up by GenericNickname
Created: 2015-12-07
Problem
There's a lot of images here, like, a lot. But they have significance.
Each image contains a punchagle, similar to the punch cards used in old computers. You're given that each shape represents a number, 0-9, and when 1 to 3 shapes in a row are converted to numbers, they make an ASCII character decimal value.
0 = filled circle
1 = outlined circle
2 = filled square
3 = outline square
4 = filled triangle
5 = outlined triangle
6 = filled semicircle UP
7 = outlined semicircle UP
8 = filled semicircle DOWN
9 = outlined semicircle DOWN
The flag is hidden somewhere in those, convert all the images and find it. sctf{FLAG} is the format.
Files
Hint
Imaging libraries might help.
Answer
Overview
Read the symbols off the images as their decimal values and then locate the flag in the output, probably with the aid of an imaging library (as indicated by the hint).
Details
There are many imaging libraries, but I decided to go with Pillow, which is for Python (Python 2.7 in my case). After examining some images, I realized it would be possible to just go through and match the numbers at the top of the images and then go through row by row and match the symbols. Pillow allowed me to do direct image evaluation, so it was just a matter of isolating the numbers and symbols, then trying to match them.
I made this image by copying the symbols out of the triangles so I would have something to match against:
To get the symbols into python, I loaded them as two arrays:
symbols = []
numerals = []
symbolBase = Image.open("punchagle_symbols.png")
for x in range(0, 10) :
symbols.append(symbolBase.crop((x * 11, 0, x * 11 + 11, 11)))
for x in range(0, 10) :
offset = 0
#this offset is due to how it was easiest to recognize the numbers;
#all numbers other than 1 and 4 only took up 5 pixels, so I special-cased 1 and 4
if x == 4 or x == 1 : offset = -1
numerals.append(symbolBase.crop((x * 6 + 1 + offset, 11, x * 6 + 6 + offset, 18)))
After making my list of symbols, I wrote a function that would tell me what symbol index a particular image matched to, or -1 if it couldn't find a match:
def getImageSymbol(syms, img) :
for x in range(0, len(syms)) :
if(syms[x] == img) :
return x
return -1
The nice thing about the triangles in this set is that it was very easy to determine where numbers and symbols were in the triangles, so I used a lot of hard coded values to read them.
I used this function to read how many rows were in the triangles by checking first for 1 digit, and if it didn't match, then reading for 2 digits:
#show flag is for debugging, can be ignored in normal running
def getRowCount(numerals, test, show = False) :
#attempt to find 1 number row count
num = test.crop((test.width / 2 - 3, 32-7, test.width / 2 + 2, 32))
numRows = getImageSymbol(numerals, num)
if (numRows == -1) :
#if 1 number isn't readable, get row count from 2 numbers
num1 = test.crop((test.width / 2 - 6, 32-7, test.width / 2 - 1, 32))
num2 = test.crop((test.width / 2, 32-7, test.width / 2 + 5, 32))
n1 = getImageSymbol(numerals, num1) * 10
n2 = getImageSymbol(numerals, num2)
if(n1 == -1 or n2 == -1) :
if show:
num.show()
num1.show()
num2.show()
return -1
numRows = n1 + n2
return numRows
Once I knew how many rows there were, I went through each row and read the symbols off it with this function:
def readTriangleRow(symbols, img, row) :
x1 = img.width / 2 - 15 - (row - 1) * 10
y1 = 40 + (row - 1) * 20
x2 = img.width / 2 + 16 + (row - 1) * 10
y2 = 51 + (row - 1) * 20
rowSyms = img.crop((x1, y1, x2, y2))
symbolCount = row + 1
tmp = ""
res = ""
for x in range(0, symbolCount) :
sym = getImageSymbol(symbols, rowSyms.crop((20 * x, 0, 20 * x + 11, 11)))
#this if can be ignored for now, it will be helpful for Punchagle 2 where symbols are omitted
if(sym == -1) : tmp += 'x'
else : tmp += str(sym)
return tmp
The rest of my program was a funtion for reading the triangles and walking through the directory to read all the images:
def readTriangle(symbols, numerals, img) :
rows = getRowCount(numerals, img)
res = ""
for x in range(1, rows + 1) :
res += readTriangleRow(symbols, img, x)
return res
out = open('symbolOutput.txt', 'w')
import os
rootdir = 'images'
count = 0
result = ""
for subdir, dirs, files in os.walk(rootdir):
for f in files:
count += 1
if count % 100 == 0 : print count #to see how far we've gotten
test = Image.open(os.path.join(subdir, f))
result += '\nTriangle: ' + f + '\n'
result += readTriangle(symbols, numerals, test)
out.write(result)
out.close()
The entire program can be downloaded here: image_symbols.py
Now that I had symbolOutput.txt, I could search for the ASCII values that should show up where the flag is. The problem says that the flag starts with sctf{ so I looked for 11599116102123 in the output and identified the end by 125, which is }. The entire string of digits was 1159911610212311211410111611612199111111108114105116101125, which, when turned into ASCII yields the flag: sctf{prettycoolrite}
Flag
sctf{prettycoolrite}
Extra Information
This solution would not have worked if the flag had been across triangles, although it could have been modified to work by removing result += '\nTriangle: ' + f + '\n' from the output and ensuring that the file system was being read in the correct order.