SquareCTF challenge 3 ("Decode me") solution

squarecrypt3.py

#!/usr/bin/env python3

import base64, string, sys

B64 = string.ascii_uppercase + string.ascii_lowercase + string.digits + '+/='
B64 = [ord(c) for c in B64]

def decode(inp):
    inp += b"\x00"

    bag = []
    buf = []
    out = []

    for x in inp:
        if x==0:
            for b in bag:
                diff_count = buf.count(b)

                ch = b+diff_count+1 - 100

                while ch not in B64:
                    ch += 100

                out.append(chr(ch))

            bag, buf = [],[]
        elif x in bag:
            buf.append(x)
        else:
            bag.append(x)

    return "".join(out)

if __name__ == '__main__':
    with open(sys.argv[1], 'rb') as f:
        print(decode(f.read()))