1 files changed, 69 insertions, 0 deletions
diff --git a/lib/math/numbertheory/powm_fw.cpp b/lib/math/numbertheory/powm_fw.cpp
new file mode 100644
index 000000000..16a48a5b0
--- /dev/null
+++ b/lib/math/numbertheory/powm_fw.cpp
@@ -0,0 +1,69 @@
+/*
+* Fixed Window Exponentiation
+* (C) 1999-2007 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#include <botan/internal/def_powm.h>
+#include <botan/numthry.h>
+#include <vector>
+
+namespace Botan {
+
+/*
+* 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 = Power_Mod::window_bits(exp.bits(), base.bits(), hints);
+
+   g.resize((1 << window_bits));
+   g[0] = 1;
+   g[1] = base;
+
+   for(size_t i = 2; i != g.size(); ++i)
+      g[i] = reducer.multiply(g[i-1], g[0]);
+   }
+
+/*
+* Compute the result
+*/
+BigInt Fixed_Window_Exponentiator::execute() const
+   {
+   const size_t exp_nibbles = (exp.bits() + window_bits - 1) / window_bits;
+
+   BigInt x = 1;
+
+   for(size_t i = exp_nibbles; i > 0; --i)
+      {
+      for(size_t j = 0; j != window_bits; ++j)
+         x = reducer.square(x);
+
+      const u32bit nibble = exp.get_substring(window_bits*(i-1), window_bits);
+
+      x = reducer.multiply(x, g[nibble]);
+      }
+   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;
+   }
+
+}