Simplify (and very very slightly speed up) the pbkdf2 implementation

    outer.update(password.translate(hmac.trans_5C))

    def F(i):

        def U():

        u = salt + struct.pack(b'>I', i)

        result = 0

        for j in xrange(int(iterations)):

            dig1, dig2 = inner.copy(), outer.copy()

            dig1.update(u)

            dig2.update(dig1.digest())

            u = dig2.digest()

                yield _bin_to_long(u)

        return _long_to_bin(reduce(operator.xor, U()), hex_format_string)

            result ^= _bin_to_long(u)

        return _long_to_bin(result, hex_format_string)

    T = [F(x) for x in range(1, l + 1)]

    return b''.join(T[:-1]) + T[-1][:r]

    T = [F(x) for x in range(1, l)]

    return b''.join(T) + F(l)[:r]