twctf-superexpress.py

#!/usr/bin/python

# shopping:
# laundry detergent

import string
import sys

key = [0] * 27
data = "805eed80cbbccb94c36413275780ec94a857dfec8da8ca94a8c313a8ccf9"
data_split = [int(data[x:x+2],16) for x in range(0,len(data),2)]

print data_split

true_a = 0
true_b = 0

print " [!] brute forcing key..."
for a in range(0,255):
  for b in range(0,255):
    c1 = (a * ord('T') + b) % 251
    c2 = (a * ord('W') + b) % 251
    c3 = (a * ord('C') + b) % 251
    if c1 == 0x80 and c2 == 0x5e and c3 == 0xed:
      print "a = %d, b = %d" % (a,b)
      true_a = a
      true_b = b
      break
  if true_a != 0 and true_b != 0:
    break

print (true_a * ord('T') + true_b) % 251

# sys.exit(0)

out = ""
print " [!] decoding ciphertext with keys %d, %d" % (true_a,true_b)
for x in range(0,len(data_split)):
  for guess in string.printable:
    if (true_a * ord(guess) + true_b) % 251 == data_split[x]:
      # print " [+] OK: %c" % guess
      out += guess

print out