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/*
* Fixed Window Exponentiation
* (C) 1999-2007 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/internal/def_powm.h>
#include <vector>
namespace Botan {
/*
* Set the exponent
*/
void Fixed_Window_Exponentiator::set_exponent(const BigInt& e)
{
m_exp = e;
}
/*
* Set the base
*/
void Fixed_Window_Exponentiator::set_base(const BigInt& base)
{
m_window_bits = Power_Mod::window_bits(m_exp.bits(), base.bits(), m_hints);
m_g.resize(static_cast<size_t>(1) << m_window_bits);
m_g[0] = 1;
m_g[1] = base;
for(size_t i = 2; i != m_g.size(); ++i)
m_g[i] = m_reducer.multiply(m_g[i-1], m_g[1]);
}
/*
* Compute the result
*/
BigInt Fixed_Window_Exponentiator::execute() const
{
const size_t exp_nibbles = (m_exp.bits() + m_window_bits - 1) / m_window_bits;
BigInt x = 1;
for(size_t i = exp_nibbles; i > 0; --i)
{
for(size_t j = 0; j != m_window_bits; ++j)
x = m_reducer.square(x);
const uint32_t nibble = m_exp.get_substring(m_window_bits*(i-1), m_window_bits);
x = m_reducer.multiply(x, m_g[nibble]);
}
return x;
}
/*
* Fixed_Window_Exponentiator Constructor
*/
Fixed_Window_Exponentiator::Fixed_Window_Exponentiator(const BigInt& n,
Power_Mod::Usage_Hints hints)
: m_reducer{Modular_Reducer(n)}, m_exp{}, m_window_bits{}, m_g{}, m_hints{hints}
{}
}
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