Nebula - Level 14 - Intuitive Encryption

About

This program resides in /home/flag14/flag14. It encrypts input and writes it to standard output. An encrypted token file is also in that home directory, decrypt it :) To do this level, log in as the level14 account with the password level14. Files for this level can be found in /home/flag14.

Solution

Entering the line echo "0123456789" | /home/flag14/flag14 -e returns 02468:<>@B. Examining the ASCII table with man ascii gives us the follwing insight.

The Difference column shows that the character shifts based on the index in which it occurs in the input. Armed with this knowledge, create a program to reverse the algorithm.

#!/usr/bin/python

tokenFile = open("/home/flag14/token", "r")
token = tokenFile.readlines()[0]
tmp = ''
index = 0
for char in token:
	if 0 <= ord(char) - index <= 255:
		tmp += chr(ord(char) - index)
		index += 1
		print tmp, index
print tmp

Save the file as decrypt.py, run chmod +x decrypt.py, and then execute the file with ./decrypt.py. This token serves as the password for the flag14 account.