1 files changed, 102 insertions, 0 deletions
diff --git a/src/math/numbertheory/powm_fw.cpp b/src/math/numbertheory/powm_fw.cpp
new file mode 100644
index 000000000..c29b9f311
--- /dev/null
+++ b/src/math/numbertheory/powm_fw.cpp
@@ -0,0 +1,102 @@
+/*************************************************
+* Fixed Window Exponentiation Source File        *
+* (C) 1999-2007 Jack Lloyd                       *
+*************************************************/
+
+#include <botan/def_powm.h>
+#include <botan/numthry.h>
+#include <vector>
+
+namespace Botan {
+
+namespace {
+
+/*************************************************
+* Try to choose a good window size               *
+*************************************************/
+u32bit choose_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                               *
+*************************************************/
+void Fixed_Window_Exponentiator::set_exponent(const BigInt& e)
+   {
+   exp = e;
+   }
+
+/*************************************************
+* Set the base                                   *
+*************************************************/
+void Fixed_Window_Exponentiator::set_base(const BigInt& base)
+   {
+   window_bits = choose_window_bits(exp.bits(), base.bits(), hints);
+
+   g.resize((1 << window_bits) - 1);
+   g[0] = base;
+   for(u32bit j = 1; j != g.size(); ++j)
+      g[j] = reducer.multiply(g[j-1], g[0]);
+   }
+
+/*************************************************
+* Compute the result                             *
+*************************************************/
+BigInt Fixed_Window_Exponentiator::execute() const
+   {
+   const u32bit exp_nibbles = (exp.bits() + window_bits - 1) / window_bits;
+
+   BigInt x = 1;
+   for(u32bit j = exp_nibbles; j > 0; --j)
+      {
+      for(u32bit k = 0; k != window_bits; ++k)
+         x = reducer.square(x);
+
+      u32bit nibble = exp.get_substring(window_bits*(j-1), window_bits);
+      if(nibble)
+         x = reducer.multiply(x, g[nibble-1]);
+      }
+   return x;
+   }
+
+/*************************************************
+* Fixed_Window_Exponentiator Constructor         *
+*************************************************/
+Fixed_Window_Exponentiator::Fixed_Window_Exponentiator(const BigInt& n,
+   Power_Mod::Usage_Hints hints)
+   {
+   reducer = Modular_Reducer(n);
+   this->hints = hints;
+   window_bits = 0;
+   }
+
+}