1 files changed, 51 insertions, 0 deletions
diff --git a/libtommath/bn_mp_mod_2d.c b/libtommath/bn_mp_mod_2d.c
new file mode 100644
index 0000000..589e4ba
--- /dev/null
+++ b/libtommath/bn_mp_mod_2d.c
@@ -0,0 +1,51 @@
+#include <tommath.h>
+#ifdef BN_MP_MOD_2D_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
+ */
+
+/* calc a value mod 2**b */
+int
+mp_mod_2d (mp_int * a, int b, mp_int * c)
+{
+  int     x, res;
+
+  /* if b is <= 0 then zero the int */
+  if (b <= 0) {
+    mp_zero (c);
+    return MP_OKAY;
+  }
+
+  /* if the modulus is larger than the value than return */
+  if (b >= (int) (a->used * DIGIT_BIT)) {
+    res = mp_copy (a, c);
+    return res;
+  }
+
+  /* copy */
+  if ((res = mp_copy (a, c)) != MP_OKAY) {
+    return res;
+  }
+
+  /* zero digits above the last digit of the modulus */
+  for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
+    c->dp[x] = 0;
+  }
+  /* clear the digit that is not completely outside/inside the modulus */
+  c->dp[b / DIGIT_BIT] &=
+    (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
+  mp_clamp (c);
+  return MP_OKAY;
+}
+#endif