CTFlearn-Writeups/Cryptography/5x5 Crypto/README.md

5x5 Crypto

The main idea finding the flag is by simple pairs.

Step-1:

After reading the given text:

Ever heard of the 5x5 secret message system? If not, basically it's a 5x5 grid with all letters of the alphabet in order, without k because c is represented to make the k sound only. Google it if you need to. A letter is identified by Row-Column. All values are in caps. Try: 1-3,4-4,2-1,{,4-4,2-3,4-5,3-2,1-2,4-3,_,4-5,3-5,}

Step-2:

So, basically the table is:

We can decode the message according to given inputs in message above to get the flag.

Step-3:

Instead I wrote a Flag.py script to get the flag.

import numpy as np

arr = np.arange(65,90).reshape(5, 5) # Array of alphabet
arr = np.where(arr <75, arr, arr+1)  # Array without 'K' 

cells = ["1-3","4-4","2-1","{","4-4","2-3","4-5","3-2","1-2","4-3","_","4-5","3-5","}"] 
for i in cells:
	if(ord(i[0])>=48 and ord(i[0])<=57):
		x=int(i[0])-1
		y=int(i[2])-1
		print(chr(arr[x][y]),end="")
	else:
		print(i[0],end="")
print("\n")

Step-4:

If we execute this script by python3 Flag.py, I got the following output:

CTF{THUMBS_UP}

Step-5:

Finally the flag becomes: CTF{THUMBS_UP}