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diff --git a/libtommath/bn_mp_mod_2d.c b/libtommath/bn_mp_mod_2d.c
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+#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