#!/usr/bin/env python3

import sys
from functools import reduce, partial

# DES functions deal with key blocks and data blocks

def encrypt(input, key):
	return cipher(input, key, range(1, 17))

def decrypt(input, key):
	return cipher(input, key, reversed(range(1, 17)))

def cipher(input, key, round_list):
	L, R = split(64, permute(64, input, IP))
	for subkey in map(partial(schedule, key), round_list):
		L, R = R, L ^ feistel(R, subkey)
	return permute(64, glue([R, L], 32), FP)

def schedule(key, round):
	CD = split(56, permute(64, key, PC1))
	CD = map(partial(rotate, 28, offset=sum(SHIFT[:round])), CD)
	return permute(56, glue(CD, 28), PC2)

def feistel(right, subkey):
	helper = partial(carve, 48, subkey ^ permute(32, right, E))
	sextets = [helper(i + 1, i + 6) for i in range(0, 48, 6)]
	nibbles = [substitute(*pair) for pair in enumerate(sextets)]
	return permute(32, glue(nibbles, 4), P)

def substitute(index, sextet):
	row = glue([head(6, sextet, 1), tail(6, sextet, 1)])
	return S[index][row][carve(6, sextet, 2, 5)]

# key derivation function (octet precision, big endian, PKCS #5)

def derive_key(file):
	# this function is far from ideal!
	return int.from_bytes(pkcs5_pad(file.read(8)), 'big')

# block cipher functions (ECB)

def ecb_encrypt(input_file, output_file, key):
	for block in plaintext_blocks(input_file):
		block = encrypt(block, key)
		ciphertext_write(output_file, block)

def ecb_decrypt(input_file, output_file, key):
	for block, finished in ciphertext_blocks(input_file):
		block = decrypt(block, key)
		plaintext_write(output_file, block, finished)

# file I/O functions (octet precision, big endian, PKCS #5)

def plaintext_blocks(file):
	while True:
		input = file.read(8)
		if len(input) == 0: break
		else: yield int.from_bytes(pkcs5_pad(input), 'big')

def ciphertext_blocks(file):
	finished, current, next = False, None, None
	while not finished:
		if current is not None:
			block = int.from_bytes(current, 'big')
			if len(next) < 8: finished = True
			yield (block, finished)
		current, next = next, file.read(8)

def plaintext_write(file, block, finished):
	sequence = block.to_bytes(8, 'big')
	if finished: sequence = pkcs5_unpad(sequence)
	file.write(sequence)

def ciphertext_write(file, block):
	file.write(block.to_bytes(8, 'big'))

# these padding functions (PKCS #5) deal with octets

def pkcs5_pad(input):
	return input + bytes([8 - len(input)] * (8 - len(input)))

def pkcs5_unpad(input):
	return input[:(8 - input[-1])]

# bitwise functions (MSB is #1)

def carve(width, input, from_index, to_index):
	mask = (1 << 1 + to_index - from_index) - 1
	return mask & input >> width - to_index

def until(width, input, index):
	return carve(width, input, 1, index)

def after(width, input, index):
	return carve(width, input, 1 + index, width)

def head(width, input, count):
	return until(width, input, count)

def tail(width, input, count):
	return after(width, input, width - count)

def upper(width, input):
	return head(width, input, width // 2)

def lower(width, input):
	return tail(width, input, width // 2)

def split(width, input):
	return pam(laitrap([upper, lower], width), input)

def glue(bits, width=1):
	return reduce(lambda value, bit: value << width | bit, bits)

def rotate(width, input, offset):
	left = after(width, input, offset) << offset
	return (tail(width + offset, left, width) |
	        until(width, input, offset))

def permute(width, input, table):
	return glue([1 & input >> width - offset for offset in table])

# functional programming

def pam(functions, *args, **kwargs):
	"""Apply functions to one list of arguments."""
	return map(lambda f: f(*args, **kwargs), functions)

def laitrap(functions, *args, **kwargs):
	"""Partially apply functions to one list of arguments."""
	return map(lambda f: partial(f, *args, **kwargs), functions)

# constants for DES from FIPS 46-3

IP = [
	58, 50, 42, 34, 26, 18, 10, 2,
	60, 52, 44, 36, 28, 20, 12, 4,
	62, 54, 46, 38, 30, 22, 14, 6,
	64, 56, 48, 40, 32, 24, 16, 8,
	57, 49, 41, 33, 25, 17, 9, 1,
	59, 51, 43, 35, 27, 19, 11, 3,
	61, 53, 45, 37, 29, 21, 13, 5,
	63, 55, 47, 39, 31, 23, 15, 7
]

FP = [
	40, 8, 48, 16, 56, 24, 64, 32,
	39, 7, 47, 15, 55, 23, 63, 31,
	38, 6, 46, 14, 54, 22, 62, 30,
	37, 5, 45, 13, 53, 21, 61, 29,
	36, 4, 44, 12, 52, 20, 60, 28,
	35, 3, 43, 11, 51, 19, 59, 27,
	34, 2, 42, 10, 50, 18, 58, 26,
	33, 1, 41, 9, 49, 17, 57, 25
]

E = [
	32, 1, 2, 3, 4, 5,
	4, 5, 6, 7, 8, 9,
	8, 9, 10, 11, 12, 13,
	12, 13, 14, 15, 16, 17,
	16, 17, 18, 19, 20, 21,
	20, 21, 22, 23, 24, 25,
	24, 25, 26, 27, 28, 29,
	28, 29, 30, 31, 32, 1
]

S = [
	[[14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7],
	 [0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8],
	 [4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0],
	 [15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13]],
	[[15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10],
	 [3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5],
	 [0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15],
	 [13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9]],
	[[10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8],
	 [13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1],
	 [13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7],
	 [1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12]],
	[[7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15],
	 [13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9],
	 [10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4],
	 [3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14]],
	[[2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9],
	 [14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6],
	 [4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14],
	 [11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3]],
	[[12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11],
	 [10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8],
	 [9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6],
	 [4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13]],
	[[4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1],
	 [13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6],
	 [1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2],
	 [6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12]],
	[[13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7],
	 [1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2],
	 [7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8],
	 [2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11]]
]

P = [
	16, 7, 20, 21,
	29, 12, 28, 17,
	1, 15, 23, 26,
	5, 18, 31, 10,
	2, 8, 24, 14,
	32, 27, 3, 9,
	19, 13, 30, 6,
	22, 11, 4, 25
]

PC1 = [
	57, 49, 41, 33, 25, 17, 9,
	1, 58, 50, 42, 34, 26, 18,
	10, 2, 59, 51, 43, 35, 27,
	19, 11, 3, 60, 52, 44, 36,
	63, 55, 47, 39, 31, 23, 15,
	7, 62, 54, 46, 38, 30, 22,
	14, 6, 61, 53, 45, 37, 29,
	21, 13, 5, 28, 20, 12, 4
]

PC2 = [
	14, 17, 11, 24, 1, 5,
	3, 28, 15, 6, 21, 10,
	23, 19, 12, 4, 26, 8,
	16, 7, 27, 20, 13, 2,
	41, 52, 31, 37, 47, 55,
	30, 40, 51, 45, 33, 48,
	44, 49, 39, 56, 34, 53,
	46, 42, 50, 36, 29, 32
]

SHIFT = [
	1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1
]

# the program

def main():
	if len(sys.argv) != 3:
		die('Usage: des.py encrypt|decrypt <key file>')
	key = derive_key(open(sys.argv[2], 'rb'))
	actions = { 'encrypt': ecb_encrypt, 'decrypt': ecb_decrypt }
	actions[sys.argv[1]](sys.stdin.buffer, sys.stdout.buffer, key)

def die(message, status=1):
	print(message, file=sys.stderr)
	sys.exit(status)

if __name__ == '__main__':
	main()