We consider the problem of finding cryptographically suitable Jacobians. By applying a probabilistic generic algorithm to compute the zeta functions of low genus curves drawn from an arbitrary family, we can search for Jacobians containing a large subgroup of prime order. For a suitable distribution of curves, the complexity is subexponential in genus 2, and O(N^{ 1/12}) in genus 3. We give examples of genus 2 and genus 3 hyperelliptic curves over prime fields with group orders over 180 bits in size, improving previous results. Our approach is particularly effective over low-degree extension fields, where in genus 2 we find Jacobians over F_{ p^2) and trace zero varieties over F_{ p^3} with near-prime orders up to 372 bits in size. For p = 2^{ 61}-1, the average time to find a group with 244-bit near-prime order is under an hour on a PC.
Categories: math.NT math.AG