--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_s_mp_invmod_fast.c	Tue May 26 17:36:47 2020 +0200
@@ -0,0 +1,118 @@
+#include "tommath_private.h"
+#ifdef BN_S_MP_INVMOD_FAST_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* computes the modular inverse via binary extended euclidean algorithm,
+ * that is c = 1/a mod b
+ *
+ * Based on slow invmod except this is optimized for the case where b is
+ * odd as per HAC Note 14.64 on pp. 610
+ */
+mp_err s_mp_invmod_fast(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   mp_int  x, y, u, v, B, D;
+   mp_sign neg;
+   mp_err  err;
+
+   /* 2. [modified] b must be odd   */
+   if (MP_IS_EVEN(b)) {
+      return MP_VAL;
+   }
+
+   /* init all our temps */
+   if ((err = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) {
+      return err;
+   }
+
+   /* x == modulus, y == value to invert */
+   if ((err = mp_copy(b, &x)) != MP_OKAY)                         goto LBL_ERR;
+
+   /* we need y = |a| */
+   if ((err = mp_mod(a, b, &y)) != MP_OKAY)                       goto LBL_ERR;
+
+   /* if one of x,y is zero return an error! */
+   if (MP_IS_ZERO(&x) || MP_IS_ZERO(&y)) {
+      err = MP_VAL;
+      goto LBL_ERR;
+   }
+
+   /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
+   if ((err = mp_copy(&x, &u)) != MP_OKAY)                        goto LBL_ERR;
+   if ((err = mp_copy(&y, &v)) != MP_OKAY)                        goto LBL_ERR;
+   mp_set(&D, 1uL);
+
+top:
+   /* 4.  while u is even do */
+   while (MP_IS_EVEN(&u)) {
+      /* 4.1 u = u/2 */
+      if ((err = mp_div_2(&u, &u)) != MP_OKAY)                    goto LBL_ERR;
+
+      /* 4.2 if B is odd then */
+      if (MP_IS_ODD(&B)) {
+         if ((err = mp_sub(&B, &x, &B)) != MP_OKAY)               goto LBL_ERR;
+      }
+      /* B = B/2 */
+      if ((err = mp_div_2(&B, &B)) != MP_OKAY)                    goto LBL_ERR;
+   }
+
+   /* 5.  while v is even do */
+   while (MP_IS_EVEN(&v)) {
+      /* 5.1 v = v/2 */
+      if ((err = mp_div_2(&v, &v)) != MP_OKAY)                    goto LBL_ERR;
+
+      /* 5.2 if D is odd then */
+      if (MP_IS_ODD(&D)) {
+         /* D = (D-x)/2 */
+         if ((err = mp_sub(&D, &x, &D)) != MP_OKAY)               goto LBL_ERR;
+      }
+      /* D = D/2 */
+      if ((err = mp_div_2(&D, &D)) != MP_OKAY)                    goto LBL_ERR;
+   }
+
+   /* 6.  if u >= v then */
+   if (mp_cmp(&u, &v) != MP_LT) {
+      /* u = u - v, B = B - D */
+      if ((err = mp_sub(&u, &v, &u)) != MP_OKAY)                  goto LBL_ERR;
+
+      if ((err = mp_sub(&B, &D, &B)) != MP_OKAY)                  goto LBL_ERR;
+   } else {
+      /* v - v - u, D = D - B */
+      if ((err = mp_sub(&v, &u, &v)) != MP_OKAY)                  goto LBL_ERR;
+
+      if ((err = mp_sub(&D, &B, &D)) != MP_OKAY)                  goto LBL_ERR;
+   }
+
+   /* if not zero goto step 4 */
+   if (!MP_IS_ZERO(&u)) {
+      goto top;
+   }
+
+   /* now a = C, b = D, gcd == g*v */
+
+   /* if v != 1 then there is no inverse */
+   if (mp_cmp_d(&v, 1uL) != MP_EQ) {
+      err = MP_VAL;
+      goto LBL_ERR;
+   }
+
+   /* b is now the inverse */
+   neg = a->sign;
+   while (D.sign == MP_NEG) {
+      if ((err = mp_add(&D, b, &D)) != MP_OKAY)                   goto LBL_ERR;
+   }
+
+   /* too big */
+   while (mp_cmp_mag(&D, b) != MP_LT) {
+      if ((err = mp_sub(&D, b, &D)) != MP_OKAY)                   goto LBL_ERR;
+   }
+
+   mp_exch(&D, c);
+   c->sign = neg;
+   err = MP_OKAY;
+
+LBL_ERR:
+   mp_clear_multi(&x, &y, &u, &v, &B, &D, NULL);
+   return err;
+}
+#endif