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__ = 'aron'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 newdef 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_halfdef 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 newdef 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')
Simplified DES encryption
python;beginner;reinventing the wheel;cryptography
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 newOf 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.