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/*
* Modular Exponentiation Proxy
* (C) 1999-2007 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#include <botan/pow_mod.h>
#include <botan/libstate.h>
#include <botan/engine.h>
namespace Botan {
/*
* Power_Mod Constructor
*/
Power_Mod::Power_Mod(const BigInt& n, Usage_Hints hints)
{
core = 0;
set_modulus(n, hints);
}
/*
* Power_Mod Copy Constructor
*/
Power_Mod::Power_Mod(const Power_Mod& other)
{
core = 0;
if(other.core)
core = other.core->copy();
}
/*
* Power_Mod Assignment Operator
*/
Power_Mod& Power_Mod::operator=(const Power_Mod& other)
{
delete core;
core = 0;
if(other.core)
core = other.core->copy();
return (*this);
}
/*
* Power_Mod Destructor
*/
Power_Mod::~Power_Mod()
{
delete core;
}
/*
* Set the modulus
*/
void Power_Mod::set_modulus(const BigInt& n, Usage_Hints hints) const
{
delete core;
core = 0;
if(n != 0)
{
Algorithm_Factory::Engine_Iterator i(global_state().algorithm_factory());
while(const Engine* engine = i.next())
{
core = engine->mod_exp(n, hints);
if(core)
break;
}
if(!core)
throw Lookup_Error("Power_Mod: Unable to find a working engine");
}
}
/*
* Set the base
*/
void Power_Mod::set_base(const BigInt& b) const
{
if(b.is_zero() || b.is_negative())
throw Invalid_Argument("Power_Mod::set_base: arg must be > 0");
if(!core)
throw Internal_Error("Power_Mod::set_base: core was NULL");
core->set_base(b);
}
/*
* Set the exponent
*/
void Power_Mod::set_exponent(const BigInt& e) const
{
if(e.is_negative())
throw Invalid_Argument("Power_Mod::set_exponent: arg must be > 0");
if(!core)
throw Internal_Error("Power_Mod::set_exponent: core was NULL");
core->set_exponent(e);
}
/*
* Compute the result
*/
BigInt Power_Mod::execute() const
{
if(!core)
throw Internal_Error("Power_Mod::execute: core was NULL");
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 {
/*
* Choose potentially useful hints
*/
Power_Mod::Usage_Hints choose_base_hints(const BigInt& b, const BigInt& n)
{
if(b == 2)
return Power_Mod::Usage_Hints(Power_Mod::BASE_IS_2 |
Power_Mod::BASE_IS_SMALL);
const u32bit b_bits = b.bits();
const u32bit n_bits = n.bits();
if(b_bits < n_bits / 32)
return Power_Mod::BASE_IS_SMALL;
if(b_bits > n_bits / 4)
return Power_Mod::BASE_IS_LARGE;
return Power_Mod::NO_HINTS;
}
/*
* Choose potentially useful hints
*/
Power_Mod::Usage_Hints choose_exp_hints(const BigInt& e, const BigInt& n)
{
const u32bit e_bits = e.bits();
const u32bit n_bits = n.bits();
if(e_bits < n_bits / 32)
return Power_Mod::BASE_IS_SMALL;
if(e_bits > n_bits / 4)
return Power_Mod::BASE_IS_LARGE;
return Power_Mod::NO_HINTS;
}
}
/*
* Fixed_Exponent_Power_Mod Constructor
*/
Fixed_Exponent_Power_Mod::Fixed_Exponent_Power_Mod(const BigInt& e,
const BigInt& n,
Usage_Hints hints) :
Power_Mod(n, Usage_Hints(hints | EXP_IS_FIXED | choose_exp_hints(e, n)))
{
set_exponent(e);
}
/*
* Fixed_Base_Power_Mod Constructor
*/
Fixed_Base_Power_Mod::Fixed_Base_Power_Mod(const BigInt& b, const BigInt& n,
Usage_Hints hints) :
Power_Mod(n, Usage_Hints(hints | BASE_IS_FIXED | choose_base_hints(b, n)))
{
set_base(b);
}
}
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