basic-mod2
Description
A new modular challenge!
Download the message here.
Take each number mod 41 and find the modular inverse for the result. Then map to the following character set: 1-26 are the alphabet, 27-36 are the decimal digits, and 37 is an underscore.
Wrap your decrypted message in the picoCTF flag format (i.e. picoCTF{decrypted_message})
Solving
This challenge is similar to the basic_mod1 just with a different mod and mod inverse.
Without further delay, directly to the script to solve the challenge:
#!/usr/bin/env python
from sympy import mod_inverse
charset = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9","_"]
flag = "picoCTF{"
encfile = "message.txt"
print("Try to decrypt file " + encfile)
with open(encfile, 'r', encoding='utf-8') as file:
for line in file:
numbers = line.split()
for number in numbers:
flag = flag + (charset[(mod_inverse(int(number),41))-1])
print("If anything worked... this should be the flag...")
print(flag + "}")