IceCTF OverTheHill writeup

over_the_hill.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import numpy as np

alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}"
alphadict = {alphabet[n]: n for n in xrange(64)}
ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"

# m = [[54, 53, 28, 20, 54, 15, 12, 7],
#      [32, 14, 24, 5, 63, 12, 50, 52],
#      [63, 59, 40, 18, 55, 33, 17, 3],
#      [63, 34, 5, 4, 56, 10, 53, 16],
#      [35, 43, 45, 53, 12, 42, 35, 37],
#      [20, 59, 42, 10, 46, 56, 12, 61],
#      [26, 39, 27, 59, 44, 54, 23, 56],
#      [32, 31, 56, 47, 31, 2, 29, 41]]

# convert m to modular inverse matrix
# calc in http://planetcalc.com/3324/
im = np.array([[31, 44, 13, 33, 47, 24, 12, 21],
               [37, 22, 19, 37, 40, 1, 59, 55],
               [45, 59, 63, 4, 27, 63, 20, 50],
               [44, 10, 0, 9, 11, 1, 14, 16],
               [54, 47, 14, 7, 31, 25, 48, 48],
               [39, 2, 56, 48, 38, 27, 48, 34],
               [38, 1, 12, 40, 40, 13, 34, 5],
               [27, 22, 5, 21, 22, 20, 7, 59]])

flag = ''
for i in xrange(0, 64, 8):
    vec = np.array([alphadict[c] for c in ciphertext[i:i+8]])
    t = im.dot(vec)
    flag += ''.join([alphabet[n%64] for n in list(t.tolist())])

print flag