Simplified DES encryption This is one of my first Python scripts and I was wondering if it meets the correct conventions. Also are there things that you would write different? I am looking for some good comments so I can start to improve my Python code from the start. (I was not supposed to use imports here)

Here's my implementation of Simplified DES:

__author__ = 'ßaron'

FIXED_IP = [2, 6, 3, 1, 4, 8, 5, 7]
FIXED_EP = [4, 1, 2, 3, 2, 3, 4, 1]
FIXED_IP_INVERSE = [4, 1, 3, 5, 7, 2, 8, 6]
FIXED_P10 = [3, 5, 2, 7, 4, 10, 1, 9, 8, 6]
FIXED_P8 = [6, 3, 7, 4, 8, 5, 10, 9]
FIXED_P4 = [2, 4, 3, 1]

S0 = [[1, 0, 3, 2],
      [3, 2, 1, 0],
      [0, 2, 1, 3],
      [3, 1, 3, 2]]

S1 = [[0, 1, 2, 3],
      [2, 0, 1, 3],
      [3, 0, 1, 0],
      [2, 1, 0, 3]]

KEY = '0111111101'


def permutate(original, fixed_key):
    new = ''
    for i in fixed_key:
        new += original[i - 1]
    return new


def left_half(bits):
    return bits[:len(bits)/2]


def right_half(bits):
    return bits[len(bits)/2:]


def shift(bits):
    rotated_left_half = left_half(bits)[1:] + left_half(bits)[0]
    rotated_right_half = right_half(bits)[1:] + right_half(bits)[0]
    return rotated_left_half + rotated_right_half


def key1():
    return permutate(shift(permutate(KEY, FIXED_P10)), FIXED_P8)


def key2():
    return permutate(shift(shift(shift(permutate(KEY, FIXED_P10)))), FIXED_P8)


def xor(bits, key):
    new = ''
    for bit, key_bit in zip(bits, key):
        new += str(((int(bit) + int(key_bit)) % 2))
    return new


def lookup_in_sbox(bits, sbox):
    row = int(bits[0] + bits[3], 2)
    col = int(bits[1] + bits[2], 2)
    return '{0:02b}'.format(sbox[row][col])


def f_k(bits, key):
    L = left_half(bits)
    R = right_half(bits)
    bits = permutate(R, FIXED_EP)
    bits = xor(bits, key)
    bits = lookup_in_sbox(left_half(bits), S0) + lookup_in_sbox(right_half(bits), S1)
    bits = permutate(bits, FIXED_P4)
    return xor(bits, L)


def encrypt(plain_text):
    bits = permutate(plain_text, FIXED_IP)
    temp = f_k(bits, key1())
    bits = right_half(bits) + temp
    bits = f_k(bits, key2())
    print permutate(bits + temp, FIXED_IP_INVERSE)


def decrypt(cipher_text):
    bits = permutate(cipher_text, FIXED_IP)
    temp = f_k(bits, key2())
    bits = right_half(bits) + temp
    bits = f_k(bits, key1())
    print permutate(bits + temp, FIXED_IP_INVERSE)


encrypt('11101010')
decrypt('10100010')
If you're using constants that are collections you'd be better off making them tuples, not lists. Trying to modify a tuple will raise an error as they're immutable, and using the tuple syntax makes it extra clear that these are unchanging constants.

FIXED_IP = (2, 6, 3, 1, 4, 8, 5, 7)
FIXED_EP = (4, 1, 2, 3, 2, 3, 4, 1)


As Caridorc said, using str.join is faster (depending on your version). It would also allow you to make permutate just one line.

def permutate(original, fixed_key):
    return ''.join(original[i - 1] for i in fixed_key)


It would also be a bit faster to pre-convert all your values in bits and key to integers. You can use map to apply a function to every member of a list, and it's faster than doing it in a list. You could do this when creating the zip object.

def xor(bits, key):
    new = ''
    for bit, key_bit in zip(map(int, bits), map(int, key)):
        new += str(((bit + key_bit) % 2))
    return new


Of course if you wanted this could also be made into a str.join, albeit a long one:

def xor(bits, key):
    return ''.join(str(((bit + key_bit) % 2)) for bit, key_bit in
                   zip(map(int, bits), map(int, key)))


There's a lot of functions here but not a lot of documentation. Explaining what individual functions do would make your code a lot easier to read. And I suspect you have some unnecessary duplicate functions, where you could pass a parameter instead of defining a whole new one. But it's hard to know when I don't entirely understand the functions you have here.