Consolidate code for choosing a window size for fixed width window

exponentiation algorithms.

4 files changed, 41 insertions, 72 deletions

diff --git a/src/math/numbertheory/pow_mod.cpp b/src/math/numbertheory/pow_mod.cpp
index e98364fea..96c978d68 100644
--- a/src/math/numbertheory/pow_mod.cpp
+++ b/src/math/numbertheory/pow_mod.cpp
@@ -111,6 +111,38 @@ BigInt Power_Mod::execute() const
    return core->execute();
    }
 
+/*
+* Try to choose a good window size
+*/
+u32bit Power_Mod::window_bits(u32bit exp_bits, u32bit base_bits,
+                              Power_Mod::Usage_Hints hints)
+   {
+   static const u32bit wsize[][2] = {
+      { 2048, 7 }, { 1024, 6 }, { 256, 5 }, { 128, 4 }, { 64, 3 }, { 0, 0 }
+   };
+
+   u32bit window_bits = 1;
+
+   if(exp_bits)
+      {
+      for(u32bit j = 0; wsize[j][0]; ++j)
+         {
+         if(exp_bits >= wsize[j][0])
+            {
+            window_bits += wsize[j][1];
+            break;
+            }
+         }
+      }
+
+   if(hints & Power_Mod::BASE_IS_FIXED)
+      window_bits += 2;
+   if(hints & Power_Mod::EXP_IS_LARGE)
+      ++window_bits;
+
+   return window_bits;
+   }
+
 namespace {
 
 /*
diff --git a/src/math/numbertheory/pow_mod.h b/src/math/numbertheory/pow_mod.h
index 6952dcd1b..7b92f0ec4 100644
--- a/src/math/numbertheory/pow_mod.h
+++ b/src/math/numbertheory/pow_mod.h
@@ -31,6 +31,7 @@ class BOTAN_DLL Modular_Exponentiator
 class BOTAN_DLL Power_Mod
    {
    public:
+
       enum Usage_Hints {
          NO_HINTS        = 0x0000,
 
@@ -44,6 +45,12 @@ class BOTAN_DLL Power_Mod
          EXP_IS_LARGE    = 0x0400
       };
 
+      /*
+      * Try to choose a good window size
+      */
+      static u32bit window_bits(u32bit exp_bits, u32bit base_bits,
+                                Power_Mod::Usage_Hints hints);
+
       void set_modulus(const BigInt&, Usage_Hints = NO_HINTS) const;
       void set_base(const BigInt&) const;
       void set_exponent(const BigInt&) const;
diff --git a/src/math/numbertheory/powm_fw.cpp b/src/math/numbertheory/powm_fw.cpp
index 1cfcdcd66..68dabc332 100644
--- a/src/math/numbertheory/powm_fw.cpp
+++ b/src/math/numbertheory/powm_fw.cpp
@@ -11,44 +11,6 @@
 
 namespace Botan {
 
-namespace {
-
-/*
-* Try to choose a good window size
-*/
-u32bit fw_powm_window_bits(u32bit exp_bits, u32bit,
-                           Power_Mod::Usage_Hints hints)
-   {
-   static const u32bit wsize[][2] = {
-      { 2048, 7 }, { 1024, 6 }, { 256, 5 }, { 128, 4 }, { 64, 3 }, { 0, 0 }
-   };
-
-   u32bit window_bits = 3;
-
-   if(exp_bits)
-      {
-      for(u32bit j = 0; wsize[j][0]; ++j)
-         {
-         if(exp_bits >= wsize[j][0])
-            {
-            window_bits += wsize[j][1];
-            break;
-            }
-         }
-      }
-
-   if(hints & Power_Mod::EXP_IS_FIXED)
-      window_bits += 2;
-   if(hints & Power_Mod::EXP_IS_LARGE)
-      window_bits += 2;
-   if(hints & Power_Mod::BASE_IS_FIXED)
-      ++window_bits;
-
-   return window_bits;
-   }
-
-}
-
 /*
 * Set the exponent
 */
@@ -62,7 +24,7 @@ void Fixed_Window_Exponentiator::set_exponent(const BigInt& e)
 */
 void Fixed_Window_Exponentiator::set_base(const BigInt& base)
    {
-   window_bits = fw_powm_window_bits(exp.bits(), base.bits(), hints);
+   window_bits = Power_Mod::window_bits(exp.bits(), base.bits(), hints);
 
    g.resize((1 << window_bits) - 1);
    g[0] = base;
diff --git a/src/math/numbertheory/powm_mnt.cpp b/src/math/numbertheory/powm_mnt.cpp
index 2d18ccdef..cce142020 100644
--- a/src/math/numbertheory/powm_mnt.cpp
+++ b/src/math/numbertheory/powm_mnt.cpp
@@ -14,38 +14,6 @@ namespace Botan {
 namespace {
 
 /*
-* Try to choose a good window size
-*/
-u32bit montgomery_powm_window_bits(u32bit exp_bits, u32bit,
-                                   Power_Mod::Usage_Hints hints)
-   {
-   static const u32bit wsize[][2] = {
-      { 2048, 4 }, { 1024, 3 }, { 256, 2 }, { 128, 1 }, { 0, 0 }
-   };
-
-   u32bit window_bits = 1;
-
-   if(exp_bits)
-      {
-      for(u32bit j = 0; wsize[j][0]; ++j)
-         {
-         if(exp_bits >= wsize[j][0])
-            {
-            window_bits += wsize[j][1];
-            break;
-            }
-         }
-      }
-
-   if(hints & Power_Mod::BASE_IS_FIXED)
-      window_bits += 2;
-   if(hints & Power_Mod::EXP_IS_LARGE)
-      ++window_bits;
-
-   return window_bits;
-   }
-
-/*
 * Montgomery Reduction
 */
 inline void montgomery_reduce(BigInt& out, MemoryRegion<word>& z_buf,
@@ -76,7 +44,7 @@ void Montgomery_Exponentiator::set_exponent(const BigInt& exp)
 */
 void Montgomery_Exponentiator::set_base(const BigInt& base)
    {
-   window_bits = montgomery_powm_window_bits(exp.bits(), base.bits(), hints);
+   window_bits = Power_Mod::window_bits(exp.bits(), base.bits(), hints);
 
    g.resize((1 << window_bits) - 1);