# Sets the key parity bit of a DES key byte
def set_parity(b):
    # If Hamming weight is even,
    # flip least-significant bit
    if hw(b) % 2 == 0:
        return b ^ 0x01
    # Else, return b as-is.
    return b

# Take a 20-bit number and generate a DES key with
# parity bits in significant bytes, followed by
# zero-padding.
def get_key(n):
    # 20 = 2*8+4. Subtracting parity bits gives
    # effective key bits: 2*7+6, so we shift
    # each whole byte by 7 * the number of less
    # significant bytes + effective length of least
    # significant byte, take modulo 2^7 = 128 and
    # finally shift one to the left to make space
    # for the parity bit.

    # Byte 3 (least-significant byte): take modulo
    # 2^l, where l is the size of the partial byte
    # in bits, and shift by 8-l. This gives 8-6=2.
    b3 = set_parity((n % 64) << 2)
    # Byte 2: Shift by combined length of less
    # signficiant bytes: 6
    b2 = set_parity(((n >> 6) % 128) << 1)
    # Byte 3: Shift by combined length of less
    # signficiant bytes: 7+6=13
    b1 = set_parity(((n >> 13) % 128) << 1)

    # Assemble the generated bytes into a
    # bytearray object.
    key = bytearray([b1, b2, b3, 0, 0, 0, 0, 0])

    # Return the key as a bytes object for DES
    # library compatibility.
    return bytes(key)