Python source code attachment for the article at: https://penafieljlm.wordpress.com/2016/10/29/ekoparty-ctf-2016-write-ups/#web-150

creditcard.py

#
# creditcard.py
#
# Author:
# John Lawrence M. Penafiel (penafieljlm)
#
# Python source code attachment for the article at:
# https://penafieljlm.wordpress.com/2016/10/29/ekoparty-ctf-2016-write-ups/#web-150
#

import math

def cardLuhnChecksumIsValid(card_number):
    """ checks to make sure that the card passes a luhn mod-10 checksum """

    sum = 0
    num_digits = len(card_number)
    oddeven = num_digits & 1

    for count in range(0, num_digits):
        digit = int(card_number[count])

        if not (( count & 1 ) ^ oddeven ):
            digit = digit * 2
        if digit > 9:
            digit = digit - 9

        sum = sum + digit

    return ( (sum % 10) == 0 )

# list format: [missing_len, prefix, postfix]
jobs = [
    [5], [8], [7]
]

# read input formatted in the following manner:
# 4716
# rest of your number
# 6108
# Mastercard
# 5238
# rest of your number
# 5763
# American Express
# 3472
# rest of your number
# 2959
index = 0
for index in range(0, 10 + 1):
    line = raw_input()
    if index == 0 or index == 2:
        jobs[0].append(line.strip())
    if index == 4 or index == 6:
        jobs[1].append(line.strip())
    if index == 8 or index == 10:
        jobs[2].append(line.strip())
    index += 1

for job in jobs:
    length, prefix, postfix = job
    for i in range(0, int(math.pow(10, length))):
        iteration = str(i).zfill(length)
        number = prefix + iteration + postfix
        if cardLuhnChecksumIsValid(number):
            print iteration
            break