Re: Elliptic Curve Y**2 = x**3 + a * x**2 + b

[trollins@interactive.visa.com (Tom Rollins)]

Justin Card wrote:
> I can't remember the elliptic curve system well, but if the parameters
> of the curve are not standard for everyone (which I am afraid they are)
> one method is to pick the point first, then solve for the a & b.
> 
> If this is not the case, finding the square root may be nice or tricky.
> 
> if p=3 mod 4, then the sqrt is
> X^(P+1) mod P, where X is the number you are trying to find the sqrt
> of.  It can be extended to X=5(mod 8) and a few others, but I'm not sure
> how.  There is also a form for X=1 mod 4,but I can't find reference to
> it. Hope this helps

A security issue is selecting an elliptic curve whose order (number
of points on the elliptic curve) is divisible by a large prime number.

I still have to implement this selection process and thus will have
my a and b selections driven by this analysis.

There also could be some bandwidth savings when transmitting an
elliptic curve point to transmitt just the x and the sign bit of y
and let the receiver reconstruct the actual y value.

The choice for prime p could have overall speed benefits by selecting
a p=3 mod 4 that makes the math simpler. This was also in Wei Dai's
ModularSquareRoot C++ code "if(p%4 == 3) return a_exp_b_mod_c(a, (p+1)/4, p);"

-tom
                




-- 
Tom Rollins   <trollins@interactive.visa.com>