crypt and decrypt to bitmask

bitmask.py

string  = "abcdefabcdeabcddcbacdbacdbbabdcfbcdabfbcdf"
result  = []
mask    = 0
rules   = {
    1: "a",
    2: "b",
    4: "c",
    8: "d",
    16: "e",
    32: "f",
}
# removing duples in string
string = "".join(set([x for x in string]))
# adding bitmask for strings
for i,vl in iter(reversed(sorted(rules.iteritems()))):
    if vl in string:
        mask += i    

while mask > 0:
    for i,vl in iter(reversed(sorted(rules.iteritems()))):
        if mask - i >= 0:
            mask -= i
            result.append(vl)

print result