mp_expt_d: bring back pre 921be35779 state

The implementation of the expt_d functionality is now implemented in the
mp_expt_d_ex() function.

The user can now choose between the old (more timing resistant) version
and the new version by modification of the parameter 'fast'.

mp_expt_d() defaults to the old version

bn_mp_expt_d.c

@@ -15,41 +15,12 @@
  * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
  */
 
-/* calculate c = a**b  using a square-multiply algorithm */
+/* wrapper function for mp_expt_d_ex() */
 int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
 {
-  int     res;
-  mp_int  g;
-
-  if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
-    return res;
-  }
-
-  /* set initial result */
-  mp_set (c, 1);
-
-  while (b > 0) {
-    /* if the bit is set multiply */
-    if (b & 1) {
-      if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
-        mp_clear (&g);
-        return res;
-      }
-    }
-
-    /* square */
-    if (b > 1 && (res = mp_sqr (&g, &g)) != MP_OKAY) {
-      mp_clear (&g);
-      return res;
-    }
-
-    /* shift to next bit */
-    b >>= 1;
-  }
-
-  mp_clear (&g);
-  return MP_OKAY;
+  return mp_expt_d_ex(a, b, c, 0);
 }
+
 #endif
 
 /* $Source$ */

bn_mp_expt_d_ex.c

@@ -0,0 +1,81 @@
+#include <tommath.h>
+#ifdef BN_MP_EXPT_D_EX_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@gmail.com, http://libtom.org
+ */
+
+/* calculate c = a**b  using a square-multiply algorithm */
+int mp_expt_d_ex (mp_int * a, mp_digit b, mp_int * c, int fast)
+{
+  int     res;
+  unsigned int x;
+
+  mp_int  g;
+
+  if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
+    return res;
+  }
+
+  /* set initial result */
+  mp_set (c, 1);
+
+  if (fast) {
+    while (b > 0) {
+      /* if the bit is set multiply */
+      if (b & 1) {
+        if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
+          mp_clear (&g);
+          return res;
+        }
+      }
+
+      /* square */
+      if (b > 1 && (res = mp_sqr (&g, &g)) != MP_OKAY) {
+        mp_clear (&g);
+        return res;
+      }
+
+      /* shift to next bit */
+      b >>= 1;
+    }
+  }
+  else {
+    for (x = 0; x < DIGIT_BIT; x++) {
+      /* square */
+      if ((res = mp_sqr (c, c)) != MP_OKAY) {
+        mp_clear (&g);
+        return res;
+      }
+
+      /* if the bit is set multiply */
+      if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
+        if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
+           mp_clear (&g);
+           return res;
+        }
+      }
+
+      /* shift to next bit */
+      b <<= 1;
+    }
+    } /* if ... else */
+
+    mp_clear (&g);
+  return MP_OKAY;
+}
+#endif
+
+/* $Source$ */
+/* $Revision$ */
+/* $Date$ */

makefile

@@ -96,7 +96,7 @@ bn_mp_init_multi.o bn_mp_clear_multi.o bn_mp_exteuclid.o bn_mp_toradix_n.o \
 bn_mp_prime_random_ex.o bn_mp_get_int.o bn_mp_sqrt.o bn_mp_is_square.o bn_mp_init_set.o \
 bn_mp_init_set_int.o bn_mp_invmod_slow.o bn_mp_prime_rabin_miller_trials.o \
 bn_mp_to_signed_bin_n.o bn_mp_to_unsigned_bin_n.o bn_mp_import.o bn_mp_export.o \
-bn_mp_balance_mul.o
+bn_mp_balance_mul.o bn_mp_expt_d_ex.o
 
 $(LIBNAME):  $(OBJECTS)
 	$(AR) $(ARFLAGS) $@ $(OBJECTS)

tommath.h

@@ -364,6 +364,7 @@ int mp_div_3(mp_int *a, mp_int *c, mp_digit *d);
 
 /* c = a**b */
 int mp_expt_d(mp_int *a, mp_digit b, mp_int *c);
+int mp_expt_d_ex (mp_int * a, mp_digit b, mp_int * c, int fast);
 
 /* c = a mod b, 0 <= c < b  */
 int mp_mod_d(mp_int *a, mp_digit b, mp_digit *c);

tommath_class.h

@@ -41,6 +41,7 @@
 #define BN_MP_EXCH_C
 #define BN_MP_EXPORT_C
 #define BN_MP_EXPT_D_C
+#define BN_MP_EXPT_D_EX_C
 #define BN_MP_EXPTMOD_C
 #define BN_MP_EXPTMOD_FAST_C
 #define BN_MP_EXTEUCLID_C
@@ -333,6 +334,10 @@
 #endif
 
 #if defined(BN_MP_EXPT_D_C)
+   #define BN_MP_EXPT_D_EX_C
+#endif
+
+#if defined(BN_MP_EXPT_D_EX_C)
    #define BN_MP_INIT_COPY_C
    #define BN_MP_SET_C
    #define BN_MP_MUL_C