zyxel_scramble.py

#!/usr/bin/env python3
#
# zyxel_scramble.py
#
# author: Felis-Sapiens
#
# Decode password from ZyXEL config (NDMS V2)

import sys
from base64 import b64decode, b64encode
from hashlib import md5

def first_step(password, x1, x2):
    x1 &= 0xff
    x2 &= 0xff
    for i in range(len(password)):
        password[i] = (password[i] ^ x2) & 0xff
        x2, x1 = x1 + x2, x2
    return password

def second_step(password):
    x = 0
    for b in password:
        x ^= b
    for i in range(1,8):
        a = ((x >> i) ^ (~x << i)) & 0xff
        x = ((a << i) ^ (~a ^ x)) & 0xff
    for i in range(len(password)):
        password[i] ^= x
    return password


def scramble_decode(password):
    if len(password) % 4 != 0:
        password += '=' * (4 - len(password) % 4)

    password = list(b64decode(password))
    length = len(password)
    if length not in [0x12, 0x24, 0x48]:
        print('ERROR: invalid input length')
        return

    a2 = length // 8
    x2 = password[a2]
    del password[a2]

    a1 = x2 % (length - 1)
    x1 = password[a1]
    del password[a1]

    password = second_step(password)
    password = first_step(password, x1, x2)

    zero_pos = password.index(0)
    if zero_pos != -1:
        length = zero_pos
    return bytes(password[:length]).decode()

def scramble_encode(password):
    old_length = len(password)
    length = len(password)
    if length < 0x13:
        length = 0x12
    elif length < 0x25:
        length = 0x24
    elif length < 0x49:
        length = 0x48
    else:
        print('ERROR: password is too long')
        return

    password = password.encode()
    md5_digest = md5(password).digest()

    password = list(password)
    if length != old_length:
        password.append(0)
        for i in range(old_length+1, length-2):
            password.append((password[i-old_length-1] * 2 + md5_digest[2 + i%14]) & 0xff)

    x1 = md5_digest[0]
    x2 = md5_digest[1]
    password = first_step(password, x1, x2)
    password = second_step(password)
    password.insert(x2 % (length - 1), x1)
    password.insert(length // 8, x2)
    return b64encode(bytes(password))

def main():
    if len(sys.argv) < 3:
        print('Usage: zyxel_scramble.py decode|encode password')
        return

    if sys.argv[1] == 'decode':
        password = scramble_decode(sys.argv[2])
    elif sys.argv[1] == 'encode':
        password = scramble_encode(sys.argv[2])
    else:
        print('ERROR: unknown command', sys.argv[1])
        return
    if password is not None:
        print(password)

if __name__ == '__main__':
    main()