bn_mod_add_fixed_top
int ret = bn_mod_add_fixed_top(r, a, b, m);
if (!bn_mod_add_fixed_top(s, s, m, order)) {
|| !bn_mod_add_fixed_top(r0, r0, m1, rsa->n))
int bn_mod_add_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,