1 files changed, 71 insertions, 0 deletions
diff --git a/third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_sub.c b/third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_sub.c
new file mode 100644
index 0000000..5672dab
--- /dev/null
+++ b/third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_sub.c
@@ -0,0 +1,71 @@
+#include "tommath_private.h"
+#ifdef BN_S_MP_SUB_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
+mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   int    olduse, min, max;
+   mp_err err;
+
+   /* find sizes */
+   min = b->used;
+   max = a->used;
+
+   /* init result */
+   if (c->alloc < max) {
+      if ((err = mp_grow(c, max)) != MP_OKAY) {
+         return err;
+      }
+   }
+   olduse = c->used;
+   c->used = max;
+
+   {
+      mp_digit u, *tmpa, *tmpb, *tmpc;
+      int i;
+
+      /* alias for digit pointers */
+      tmpa = a->dp;
+      tmpb = b->dp;
+      tmpc = c->dp;
+
+      /* set carry to zero */
+      u = 0;
+      for (i = 0; i < min; i++) {
+         /* T[i] = A[i] - B[i] - U */
+         *tmpc = (*tmpa++ - *tmpb++) - u;
+
+         /* U = carry bit of T[i]
+          * Note this saves performing an AND operation since
+          * if a carry does occur it will propagate all the way to the
+          * MSB.  As a result a single shift is enough to get the carry
+          */
+         u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
+
+         /* Clear carry from T[i] */
+         *tmpc++ &= MP_MASK;
+      }
+
+      /* now copy higher words if any, e.g. if A has more digits than B  */
+      for (; i < max; i++) {
+         /* T[i] = A[i] - U */
+         *tmpc = *tmpa++ - u;
+
+         /* U = carry bit of T[i] */
+         u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
+
+         /* Clear carry from T[i] */
+         *tmpc++ &= MP_MASK;
+      }
+
+      /* clear digits above used (since we may not have grown result above) */
+      MP_ZERO_DIGITS(tmpc, olduse - c->used);
+   }
+
+   mp_clamp(c);
+   return MP_OKAY;
+}
+
+#endif