#!/home/maxime/.pyvenv/bin/python3 with open("message.txt", 'r') as f: text = f.read().strip() cipher = text.split(' ') dec = "" for i in cipher: i = int(i) i %= 41 i = pow(i, -1, 41) if i <= 26: dec += chr(ord('a') + i - 1) elif i <= 36: dec += chr(ord('0') + i - 27) elif i == 37: dec += '_' print(dec)