Python: Trying to speed up a program that is running very slow - python-2.7

Question

So, this program parses an e-mail address and a plain-text password from a text file. Then, it runs them through a few encryption routines and appends the encrypted text onto the end of the e-amil address:password entry in a new file.
import io
from Crypto.Cipher import AES
import base64
import struct
def str_to_a32(b):
if len(b) % 4:
b += '\0' * (4 - len(b) % 4)
return struct.unpack('>%dI' % (len(b) / 4), b)
def a32_to_str(a):
return struct.pack('>%dI' % len(a), *a)
def aes_cbc_encrypt(data, key):
encryptor = AES.new(key, AES.MODE_CBC, '\0' * 16)
return encryptor.encrypt(data)
def aes_cbc_encrypt_a32(data, key):
return str_to_a32(aes_cbc_encrypt(a32_to_str(data), a32_to_str(key)))
def base64urlencode(data):
data = base64.b64encode(data)
for search, replace in (('+', '-'), ('/', '_'), ('=', '')):
data = data.replace(search, replace)
return data
def a32_to_base64(a):
return base64urlencode(a32_to_str(a))
def stringhash(s, aeskey):
s32 = str_to_a32(s)
h32 = [0, 0, 0, 0]
for i in xrange(len(s32)):
h32[i % 4] ^= s32[i]
for _ in xrange(0x4000):
h32 = aes_cbc_encrypt_a32(h32, aeskey)
return a32_to_base64((h32[0], h32[2]))
def prepare_key(a):
pkey = [0x93C467E3, 0x7DB0C7A4, 0xD1BE3F81, 0x0152CB56]
for _ in xrange(0x10000):
for j in xrange(0, len(a), 4):
key = [0, 0, 0, 0]
for i in xrange(4):
if i + j < len(a):
key[i] = a[i + j]
pkey = aes_cbc_encrypt_a32(pkey, key)
return pkey
with io.open('user_list.txt', 'r') as file:
with io.open('user_list_enc.txt', 'a') as enc_file:
for line in file:
email_split, pass_split = line.replace('\n', '').split(":")
password_aes = prepare_key(str_to_a32(pass_split))
uh = stringhash(email_split.lower(), password_aes)
enc_file.write(email_split + ":" + pass_split + ":" + uh + "\n")
print email_split + ":" + pass_split + ":" + uh + "\n"