--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_to_ubin.c	Thu Oct 15 19:55:15 2020 +0800
@@ -0,0 +1,41 @@
+#include "tommath_private.h"
+#ifdef BN_MP_TO_UBIN_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* store in unsigned [big endian] format */
+mp_err mp_to_ubin(const mp_int *a, unsigned char *buf, size_t maxlen, size_t *written)
+{
+   size_t  x, count;
+   mp_err  err;
+   mp_int  t;
+
+   count = mp_ubin_size(a);
+   if (count > maxlen) {
+      return MP_BUF;
+   }
+
+   if ((err = mp_init_copy(&t, a)) != MP_OKAY) {
+      return err;
+   }
+
+   for (x = count; x --> 0u;) {
+#ifndef MP_8BIT
+      buf[x] = (unsigned char)(t.dp[0] & 255u);
+#else
+      buf[x] = (unsigned char)(t.dp[0] | ((t.dp[1] & 1u) << 7));
+#endif
+      if ((err = mp_div_2d(&t, 8, &t, NULL)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+   }
+
+   if (written != NULL) {
+      *written = count;
+   }
+
+LBL_ERR:
+   mp_clear(&t);
+   return err;
+}
+#endif