--- /dev/null
+#include "tommath_private.h"
+#ifdef BN_MP_MOD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* c = a mod b, 0 <= c < b if b > 0, b < c <= 0 if b < 0 */
+mp_err mp_mod(const mp_int *a, const mp_int *b, mp_int *c)
+{
+ mp_int t;
+ mp_err err;
+
+ if ((err = mp_init_size(&t, b->used)) != MP_OKAY) {
+ return err;
+ }
+
+ if ((err = mp_div(a, b, NULL, &t)) != MP_OKAY) {
+ goto LBL_ERR;
+ }
+
+ if (MP_IS_ZERO(&t) || (t.sign == b->sign)) {
+ err = MP_OKAY;
+ mp_exch(&t, c);
+ } else {
+ err = mp_add(b, &t, c);
+ }
+
+LBL_ERR:
+ mp_clear(&t);
+ return err;
+}
+#endif