--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_from_sbin.c	Fri Jun 26 21:07:34 2020 +0800
@@ -0,0 +1,25 @@
+#include "tommath_private.h"
+#ifdef BN_MP_FROM_SBIN_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* read signed bin, big endian, first byte is 0==positive or 1==negative */
+mp_err mp_from_sbin(mp_int *a, const unsigned char *buf, size_t size)
+{
+   mp_err err;
+
+   /* read magnitude */
+   if ((err = mp_from_ubin(a, buf + 1, size - 1u)) != MP_OKAY) {
+      return err;
+   }
+
+   /* first byte is 0 for positive, non-zero for negative */
+   if (buf[0] == (unsigned char)0) {
+      a->sign = MP_ZPOS;
+   } else {
+      a->sign = MP_NEG;
+   }
+
+   return MP_OKAY;
+}
+#endif