compute endomorphism orders for elliptic curves over finite fields (rank-2 case)

[yyyyx4]

For an elliptic curve E over a finite field with an endomorphism algebra of rank two over ℚ, the endomorphism ring is a superorder of the "Frobenius order" ℤ[π].

This patch adds a simple method to compute the exact imaginary-quadratic order containing ℤ[π] which is isomorphic to the endomorphism ring of the curve, and generalizes the algorithm to supersingular elliptic curves with a (rational) endomorphism algebra of rank 2.

[JohnCremona]

I'll look at this tomorrow. I did implement a special case of Sutherland 's algorithm already for our CM testing algorithm last year.

[yyyyx4]

Thank you! I think the implementation you wrote is the function I'm calling here. I just extended it to work for the supersingular case, too.