--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_s_mp_karatsuba_sqr.c	Tue May 26 17:36:47 2020 +0200
@@ -0,0 +1,110 @@
+#include "tommath_private.h"
+#ifdef BN_S_MP_KARATSUBA_SQR_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* Karatsuba squaring, computes b = a*a using three
+ * half size squarings
+ *
+ * See comments of karatsuba_mul for details.  It
+ * is essentially the same algorithm but merely
+ * tuned to perform recursive squarings.
+ */
+mp_err s_mp_karatsuba_sqr(const mp_int *a, mp_int *b)
+{
+   mp_int  x0, x1, t1, t2, x0x0, x1x1;
+   int     B;
+   mp_err  err = MP_MEM;
+
+   /* min # of digits */
+   B = a->used;
+
+   /* now divide in two */
+   B = B >> 1;
+
+   /* init copy all the temps */
+   if (mp_init_size(&x0, B) != MP_OKAY)
+      goto LBL_ERR;
+   if (mp_init_size(&x1, a->used - B) != MP_OKAY)
+      goto X0;
+
+   /* init temps */
+   if (mp_init_size(&t1, a->used * 2) != MP_OKAY)
+      goto X1;
+   if (mp_init_size(&t2, a->used * 2) != MP_OKAY)
+      goto T1;
+   if (mp_init_size(&x0x0, B * 2) != MP_OKAY)
+      goto T2;
+   if (mp_init_size(&x1x1, (a->used - B) * 2) != MP_OKAY)
+      goto X0X0;
+
+   {
+      int x;
+      mp_digit *dst, *src;
+
+      src = a->dp;
+
+      /* now shift the digits */
+      dst = x0.dp;
+      for (x = 0; x < B; x++) {
+         *dst++ = *src++;
+      }
+
+      dst = x1.dp;
+      for (x = B; x < a->used; x++) {
+         *dst++ = *src++;
+      }
+   }
+
+   x0.used = B;
+   x1.used = a->used - B;
+
+   mp_clamp(&x0);
+
+   /* now calc the products x0*x0 and x1*x1 */
+   if (mp_sqr(&x0, &x0x0) != MP_OKAY)
+      goto X1X1;           /* x0x0 = x0*x0 */
+   if (mp_sqr(&x1, &x1x1) != MP_OKAY)
+      goto X1X1;           /* x1x1 = x1*x1 */
+
+   /* now calc (x1+x0)**2 */
+   if (s_mp_add(&x1, &x0, &t1) != MP_OKAY)
+      goto X1X1;           /* t1 = x1 - x0 */
+   if (mp_sqr(&t1, &t1) != MP_OKAY)
+      goto X1X1;           /* t1 = (x1 - x0) * (x1 - x0) */
+
+   /* add x0y0 */
+   if (s_mp_add(&x0x0, &x1x1, &t2) != MP_OKAY)
+      goto X1X1;           /* t2 = x0x0 + x1x1 */
+   if (s_mp_sub(&t1, &t2, &t1) != MP_OKAY)
+      goto X1X1;           /* t1 = (x1+x0)**2 - (x0x0 + x1x1) */
+
+   /* shift by B */
+   if (mp_lshd(&t1, B) != MP_OKAY)
+      goto X1X1;           /* t1 = (x0x0 + x1x1 - (x1-x0)*(x1-x0))<<B */
+   if (mp_lshd(&x1x1, B * 2) != MP_OKAY)
+      goto X1X1;           /* x1x1 = x1x1 << 2*B */
+
+   if (mp_add(&x0x0, &t1, &t1) != MP_OKAY)
+      goto X1X1;           /* t1 = x0x0 + t1 */
+   if (mp_add(&t1, &x1x1, b) != MP_OKAY)
+      goto X1X1;           /* t1 = x0x0 + t1 + x1x1 */
+
+   err = MP_OKAY;
+
+X1X1:
+   mp_clear(&x1x1);
+X0X0:
+   mp_clear(&x0x0);
+T2:
+   mp_clear(&t2);
+T1:
+   mp_clear(&t1);
+X1:
+   mp_clear(&x1);
+X0:
+   mp_clear(&x0);
+LBL_ERR:
+   return err;
+}
+#endif