Encoding a numeric string into a shortened alphanumeric string, and back again

This is a pretty good compression:

import base64

def num_to_alpha(num):
    num = hex(num)[2:].rstrip("L")

    if len(num) % 2:
        num = "0" + num

    return base64.b64encode(num.decode('hex'))

It first turns the integer into a bytestring and then base64 encodes it. Here's the decoder:

def alpha_to_num(alpha):
    num_bytes = base64.b64decode(alpha)
    return int(num_bytes.encode('hex'), 16)

Example:

>>> num_to_alpha(20120425161608678259146181504021022591461815040210220120425161608667)
'vw4LUVm4Ea3fMnoTkHzNOlP6Z7eUAkHNdZjN2w=='
>>> alpha_to_num('vw4LUVm4Ea3fMnoTkHzNOlP6Z7eUAkHNdZjN2w==')
20120425161608678259146181504021022591461815040210220120425161608667

There are two functions that are custom (not based on base64), but produce shorter output:

chrs = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
l = len(chrs)

def int_to_cust(i):
    result = ''
    while i:
        result = chrs[i % l] + result
        i = i // l
    if not result:
        result = chrs[0]
    return result

def cust_to_int(s):
    result = 0
    for char in s:
        result = result * l + chrs.find(char)
    return result

And the results are:

>>> int_to_cust(20120425161608678259146181504021022591461815040210220120425161608667)
'9F9mFGkji7k6QFRACqLwuonnoj9SqPrs3G3fRx'
>>> cust_to_int('9F9mFGkji7k6QFRACqLwuonnoj9SqPrs3G3fRx')
20120425161608678259146181504021022591461815040210220120425161608667L

You can also shorten the generated string, if you add other characters to the chrs variable.