hwaes 31c3 CTF

hwaes.py

from socket import socket
from Crypto.Cipher import AES

Sbox = (
    0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76,
    0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0,
    0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15,
    0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75,
    0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84,
    0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF,
    0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8,
    0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2,
    0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73,
    0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB,
    0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79,
    0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08,
    0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A,
    0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E,
    0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
    0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16,
)

Rcon = (
    0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40,
    0x80, 0x1B, 0x36, 0x6C, 0xD8, 0xAB, 0x4D, 0x9A,
    0x2F, 0x5E, 0xBC, 0x63, 0xC6, 0x97, 0x35, 0x6A,
    0xD4, 0xB3, 0x7D, 0xFA, 0xEF, 0xC5, 0x91, 0x39,
)

def chunks(l, n):
    for i in xrange(0, len(l), n):
        yield l[i:i+n]

def xor(a, b):
    res = []
    for i in range(len(a)):
        res.append(a[i]^b[i])
    return res

def shift(s):
    return [s[1], s[2], s[3], s[0]]

def sub(s):
    return [Sbox[s[0]], Sbox[s[1]], Sbox[s[2]], Sbox[s[3]]]

def invert_schedule(key, round):
    """
    K(i-1, 4) = K(i, 3) XOR K(i, 4)
    K(i-1, 3) = K(i, 2) XOR K(i, 3)
    K(i-1, 2) = K(i, 1) XOR K(i, 2)
    K(i-1, 1) = K(i, 1) XOR sub(shift(K(i-1, 4)) XOR RCON(i)
    """

    prev_key = [None] * 4

    prev_key[3] = xor(key[2], key[3])
    prev_key[2] = xor(key[1], key[2])
    prev_key[1] = xor(key[0], key[1])
    prev_key[0] = xor(xor(key[0], sub(shift(prev_key[3]))), [Rcon[round], 0, 0, 0])

    return prev_key


if __name__ == '__main__':
    s = socket()
    s.connect(("188.40.18.66", 2786))
    s.recv(1024)

    s.send("flag\n")
    enc_flag = s.recv(1024).strip()
    print "[+] Encrypted flag:", enc_flag
    s.send("getkey\n")
    expanded_key = s.recv(1024).strip()
    print "[+] Expanded Key:", expanded_key

    key = [[ord(b) for b in x] for x in chunks(expanded_key.decode('hex'), 4)]

    for i in range(10, 0, -1):
        key = invert_schedule(key, i)

    key = ("".join(map(chr, [b for x in key for b in x]))).encode('hex')
    print "[+] Master key:", key

    flag = repr(AES.new(key.decode('hex'), AES.MODE_ECB).decrypt(enc_flag.decode('hex')))
    print "[+] Flag:", flag