1 files changed, 85 insertions, 0 deletions
diff --git a/libtommath/bn_s_mp_sub.c b/libtommath/bn_s_mp_sub.c
new file mode 100644
index 0000000..5b7aef9
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+++ b/libtommath/bn_s_mp_sub.c
@@ -0,0 +1,85 @@
+#include <tommath.h>
+#ifdef BN_S_MP_SUB_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.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
+ */
+
+/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
+int
+s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
+{
+  int     olduse, res, min, max;
+
+  /* find sizes */
+  min = b->used;
+  max = a->used;
+
+  /* init result */
+  if (c->alloc < max) {
+    if ((res = mp_grow (c, max)) != MP_OKAY) {
+      return res;
+    }
+  }
+  olduse = c->used;
+  c->used = max;
+
+  {
+    register mp_digit u, *tmpa, *tmpb, *tmpc;
+    register 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_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
+
+      /* 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_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
+
+      /* Clear carry from T[i] */
+      *tmpc++ &= MP_MASK;
+    }
+
+    /* clear digits above used (since we may not have grown result above) */
+    for (i = c->used; i < olduse; i++) {
+      *tmpc++ = 0;
+    }
+  }
+
+  mp_clamp (c);
+  return MP_OKAY;
+}
+
+#endif