eddsa: add support for point precomputation

[tomato42]

Significantly speed up all EdDSA operations by computing multiplication table of the generator point

[t184256]

Not a fan of some details, though you're unlikely to find them blocking either.

[t184256]

Here are two attempts to restructure the do-while awkwardness, with control flow

        assert self.__order
        precompute = []
        prime = self.__curve.p()
        doubler = PointEdwards(self.__curve, *self.__coords, self.__order * 2)
        i = 1
        while True:
            doubler.scale()
            coord_x, coord_y = doubler.x(), doubler.y()
            coord_t = coord_x * coord_y % prime
            precompute.append((coord_x, coord_y, coord_t))
            if i >= self.__order * 4:
                break
            i *= 2
            doubler = doubler.double()
        self.__precompute = precompute
        # assert self.__order * 8 < 2**len(precompute) < self.__order * 16

and with extracting scale-yield-double to a separate generator

        def double_gen(p):
            while True:
                yield p.scale()
                p = p.double()

        assert self.__order
        prime = self.__curve.p()
        p1 = PointEdwards(self.__curve, *self.__coords, self.__order * 2)
        import itertools
        self.__precompute = list(itertools.islice(
            ((p.x(), p.y(), p.x() * p.y() % prime) for p in double_gen(p1)),
            bit_length(self.__order) + 3
        ))
        # assert self.__order * 8 < 2**len(self.__precompute) < self.__order * 16

[tomato42]

yes, *self.__coords is nice syntactic sugar, unfortunately doesn't work on python2, but yeah, could be rewritten

[t184256]

r+ from me, I don't have any more good input

[tomato42]

@t184256 thanks for the review!