OpenSSL::PKey::EC::Group#cofactor

static VALUE ossl_ec_group_get_cofactor(VALUE self)
{
    VALUE bn_obj;
    BIGNUM *bn;
    EC_GROUP *group;

    GetECGroup(self, group);
    bn_obj = ossl_bn_new(BN_value_one());
    bn = GetBNPtr(bn_obj);

    if (EC_GROUP_get_cofactor(group, bn, ossl_bn_ctx) != 1)
        ossl_raise(eEC_GROUP, "EC_GROUP_get_cofactor");

    return bn_obj;
}