Counting points on genus-3 hyperelliptic curves with explicit real multiplication.

We propose a Las Vegas probabilistic algorithm to compute the zeta function of a genus-3 hyperelliptic curve defined over a finite field $Fq$, with explicit real multiplication by an order $Z[η]$ in a totally real cubic field. Our main result states that this algorithm requires an expected number of $O((log q) 6)$ bit-operations, where the constant in the $O()$ depends on the ring $Z[η]$ and on the degrees of polynomials representing the endomorphism $η$. As a proof-of-concept, we compute the zeta function of a curve defined over a 64-bit prime field, with explicit real multiplication by $Z[2 cos(2π/7)$].