The last step from Avast's reversing challenge in security session 2018

base18_decode.py

# The message I got from the previous step was the following:
# The last step: 4e 3f 4a 5f 3g 5f 3d 3b 4c 5f 2c 46 1e 43 2f 4h 34 1e 2c 6c 32 5b 32 57 31 30 32 2h
# From that and the hint that "16 is not enough" we had to figure out that it was base18 encoded 

# Decoder function from https://github.com/gg/integer_encoding
def decoder(alphabet):
    """
    Returns a decoder that decodes a base-`len(alphabet)` encoded sequence of
    alphabet elements into a positive integer.

    `alphabet`: a list of hashable elements used to encode an integer; i.e.
    `'0123456789'` is an alphabet consisting of digit characters.
    """
    base = len(alphabet)
    index = dict((v, k) for k, v in enumerate(alphabet))

    def decode(xs):
        try:
            result = 0
            for i, x in enumerate(xs[::-1]):
                result += (base ** i) * index[x]
            return result
        except KeyError:
            raise ValueError("%r is not in the alphabet %r" % (x, alphabet))

    return decode


hex_string = "4e 3f 4a 5f 3g 5f 3d 3b 4c 5f 2c 46 1e 43 2f 4h 34 1e 2c 6c 32 5b 32 57 31 30 32 2h".split(' ')
alphabet = '0123456789abcdefgh'
decode = decoder(alphabet)

final = []
for i in hex_string:
    final.append(chr(decode(i)))

print(''.join(final)) # VERiFiCATi0N K3Y: 0x8e8a7685