crypt500.py

import random
import string
import sys
import base64

alphabets = string.lowercase + string.uppercase
base64s = set(alphabets + '0123456789' + '+' + '/')

appeared = set()
for i in range(100000):
    for LEN in range(1,9):
        stra = ''
        strb = ''
        for i in range(LEN):
            stra += random.choice(alphabets)
            strb += random.choice(alphabets)

        if stra == strb:
            continue

        stra = base64.b64encode(stra)
        strb = base64.b64encode(strb)

        for a, b in zip(stra, strb):
            if a == b:
                appeared.add(a)

print ''.join(sorted(base64s - appeared))