--- a/libtommath/bn_mp_xor.c	Tue May 26 23:27:26 2020 +0800
+++ b/libtommath/bn_mp_xor.c	Tue May 26 17:36:47 2020 +0200
@@ -1,48 +1,56 @@
 #include "tommath_private.h"
 #ifdef BN_MP_XOR_C
-/* LibTomMath, multiple-precision integer library -- Tom St Denis
- *
- * LibTomMath is a library that provides multiple-precision
- * integer arithmetic as well as number theoretic functionality.
- *
- * The library was designed directly after the MPI library by
- * Michael Fromberger but has been written from scratch with
- * additional optimizations in place.
- *
- * SPDX-License-Identifier: Unlicense
- */
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
 
-/* XOR two ints together */
-int mp_xor(const mp_int *a, const mp_int *b, mp_int *c)
+/* two complement xor */
+mp_err mp_xor(const mp_int *a, const mp_int *b, mp_int *c)
 {
-   int     res, ix, px;
-   mp_int  t;
-   const mp_int *x;
+   int used = MP_MAX(a->used, b->used) + 1, i;
+   mp_err err;
+   mp_digit ac = 1, bc = 1, cc = 1;
+   mp_sign csign = (a->sign != b->sign) ? MP_NEG : MP_ZPOS;
 
-   if (a->used > b->used) {
-      if ((res = mp_init_copy(&t, a)) != MP_OKAY) {
-         return res;
+   if (c->alloc < used) {
+      if ((err = mp_grow(c, used)) != MP_OKAY) {
+         return err;
       }
-      px = b->used;
-      x = b;
-   } else {
-      if ((res = mp_init_copy(&t, b)) != MP_OKAY) {
-         return res;
-      }
-      px = a->used;
-      x = a;
    }
 
-   for (ix = 0; ix < px; ix++) {
-      t.dp[ix] ^= x->dp[ix];
+   for (i = 0; i < used; i++) {
+      mp_digit x, y;
+
+      /* convert to two complement if negative */
+      if (a->sign == MP_NEG) {
+         ac += (i >= a->used) ? MP_MASK : (~a->dp[i] & MP_MASK);
+         x = ac & MP_MASK;
+         ac >>= MP_DIGIT_BIT;
+      } else {
+         x = (i >= a->used) ? 0uL : a->dp[i];
+      }
+
+      /* convert to two complement if negative */
+      if (b->sign == MP_NEG) {
+         bc += (i >= b->used) ? MP_MASK : (~b->dp[i] & MP_MASK);
+         y = bc & MP_MASK;
+         bc >>= MP_DIGIT_BIT;
+      } else {
+         y = (i >= b->used) ? 0uL : b->dp[i];
+      }
+
+      c->dp[i] = x ^ y;
+
+      /* convert to to sign-magnitude if negative */
+      if (csign == MP_NEG) {
+         cc += ~c->dp[i] & MP_MASK;
+         c->dp[i] = cc & MP_MASK;
+         cc >>= MP_DIGIT_BIT;
+      }
    }
-   mp_clamp(&t);
-   mp_exch(c, &t);
-   mp_clear(&t);
+
+   c->used = used;
+   c->sign = csign;
+   mp_clamp(c);
    return MP_OKAY;
 }
 #endif
-
-/* ref:         HEAD -> master, tag: v1.1.0 */
-/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
-/* commit time: 2019-01-28 20:32:32 +0100 */