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/*
* Fixed Window Exponentiation
* (C) 1999-2007 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#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;
}
}
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