Burning Rangers password functions

brangers.py

ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ123456'
XOR_CONSTANT = 0x9fd43b75


def from_binary(all_bits):
    # Given a string of 50 '0' and '1' characters, return a 10 character password string
    ret = []
    for i in range(5, len(all_bits) + 1, 5):
        j = int(all_bits[i - 5:i], 2)
        ret.append(ALPHABET[j])

    return ''.join(ret)


def get_checksum(n):
    # Given the an integer representing the lower 32 bits of the decoded password,
    # return a 16 bit integer representing the password's checksum.
    # This is an implementation of CRC-16-GENIBUS
    msb_mask = 0x00008000
    xor_constant = 0x1021

    n_bytes = n.to_bytes(4, 'big')
    ret = 0xffff
    for i in range(4):
        ret ^= ((n_bytes[i] & 0xff) << 8)
        for j in range(2):
            if ((msb_mask & ret & 0xffff) == 0):
                x = ret << 1
            else:
                x = (ret & 0xffff) << 1 ^ xor_constant

            if ((msb_mask & x & 0xffff) == 0):
                x <<= 1
            else:
                x = (x & 0xffff) << 1 ^ xor_constant

            if ((msb_mask & x & 0xffff) == 0):
                x <<= 1
            else:
                x = (x & 0xffff) << 1 ^ xor_constant

            if ((msb_mask & x & 0xffff) == 0):
                ret = x << 1
            else:
                ret = ((x & 0xffff) << 1 ^ xor_constant)

    return ~ret & 0xffff


def construct_password(mission, n_15, n_12):
    # Given a mission number (1, 2, or 3), a 15 bit integer, and a 12 bit integer,
    # return a valid password string. The meaning of the two integers is currently
    # unknown. However:
    # construct_password(1, 0, 0) gives D2H5P4IO2V, the original map for mission 1.
    # construct_password(2, 0, 0) gives DYJSP4IK2V, the original map for mission 2.
    # construct_password(3, 0, 0) gives D42GP4IG2V, the original map for mission 3.
    if not (1 <= mission <= 3):
        raise ValueError('invalid mission')

    if not (0 <= n_15 < 2 ** 15):
        raise ValueError('invalid n_15')

    if not (0 <= n_12 < 2 ** 12):
        raise ValueError('invalid n_12')

    lower_bits = (
        format(n_15, '018b') + format(mission - 1, '02b') + format(n_12, '012b')
    )
    lower_int = int(lower_bits, 2)
    checksum_int = get_checksum(lower_int)
    checksum_bits = format(checksum_int, '016b')
    reconstructed_bits = '00' + checksum_bits + format(lower_int ^ XOR_CONSTANT, '032b')
    
    return from_binary(reconstructed_bits)

Here's a more straightforward implementation of the checksum function.

def crc_16_genibus(n):
     ret = 0xffff
     for d in n.to_bytes(4, 'big'):
         x = (ret >> 8) ^ d
         x ^= (x >> 4)
         ret = (ret << 8) ^ (x << 12) ^ (x << 5) ^ x
         ret &= 0xffff
     return ret ^ 0xffff