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/*
* Montgomery Exponentiation
* (C) 1999-2010 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#include <botan/internal/def_powm.h>
#include <botan/numthry.h>
#include <botan/internal/mp_core.h>
namespace Botan {
/*
* Set the exponent
*/
void Montgomery_Exponentiator::set_exponent(const BigInt& exp)
{
this->exp = exp;
exp_bits = exp.bits();
}
/*
* Set the base
*/
void Montgomery_Exponentiator::set_base(const BigInt& base)
{
window_bits = Power_Mod::window_bits(exp.bits(), base.bits(), hints);
g.resize((1 << window_bits) - 1);
SecureVector<word> z(2 * (mod_words + 1));
SecureVector<word> workspace(z.size());
g[0] = (base >= modulus) ? (base % modulus) : base;
bigint_mul(&z[0], z.size(), &workspace[0],
g[0].data(), g[0].size(), g[0].sig_words(),
R2.data(), R2.size(), R2.sig_words());
bigint_monty_redc(&z[0], z.size(),
&workspace[0],
modulus.data(), mod_words, mod_prime);
g[0].get_reg().set(&z[0], mod_words + 1);
const BigInt& x = g[0];
const size_t x_sig = x.sig_words();
for(size_t i = 1; i != g.size(); ++i)
{
const BigInt& y = g[i-1];
const size_t y_sig = y.sig_words();
zeroise(z);
bigint_mul(&z[0], z.size(), &workspace[0],
x.data(), x.size(), x_sig,
y.data(), y.size(), y_sig);
bigint_monty_redc(&z[0], z.size(),
&workspace[0],
modulus.data(), mod_words, mod_prime);
g[i].get_reg().set(&z[0], mod_words + 1);
}
}
/*
* Compute the result
*/
BigInt Montgomery_Exponentiator::execute() const
{
const size_t exp_nibbles = (exp_bits + window_bits - 1) / window_bits;
BigInt x = R_mod;
SecureVector<word> z(2 * (mod_words + 1));
SecureVector<word> workspace(2 * (mod_words + 1));
for(size_t i = exp_nibbles; i > 0; --i)
{
for(size_t k = 0; k != window_bits; ++k)
{
zeroise(z);
bigint_sqr(&z[0], z.size(), &workspace[0],
x.data(), x.size(), x.sig_words());
bigint_monty_redc(&z[0], z.size(),
&workspace[0],
modulus.data(), mod_words, mod_prime);
x.get_reg().set(&z[0], mod_words + 1);
}
size_t nibble = exp.get_substring(window_bits*(i-1), window_bits);
if(nibble)
{
const BigInt& y = g[nibble-1];
zeroise(z);
bigint_mul(&z[0], z.size(), &workspace[0],
x.data(), x.size(), x.sig_words(),
y.data(), y.size(), y.sig_words());
bigint_monty_redc(&z[0], z.size(),
&workspace[0],
modulus.data(), mod_words, mod_prime);
x.get_reg().set(&z[0], mod_words + 1);
}
}
x.get_reg().resize(2*mod_words+1);
bigint_monty_redc(&x[0], x.size(),
&workspace[0],
modulus.data(), mod_words, mod_prime);
x.get_reg().resize(mod_words+1);
return x;
}
/*
* Montgomery_Exponentiator Constructor
*/
Montgomery_Exponentiator::Montgomery_Exponentiator(const BigInt& mod,
Power_Mod::Usage_Hints hints)
{
// Montgomery reduction only works for positive odd moduli
if(!mod.is_positive() || mod.is_even())
throw Invalid_Argument("Montgomery_Exponentiator: invalid modulus");
window_bits = 0;
this->hints = hints;
modulus = mod;
mod_words = modulus.sig_words();
BigInt mod_prime_bn(BigInt::Power2, MP_WORD_BITS);
mod_prime = (mod_prime_bn - inverse_mod(modulus, mod_prime_bn)).word_at(0);
R_mod = BigInt(BigInt::Power2, MP_WORD_BITS * mod_words);
R_mod %= modulus;
R2 = BigInt(BigInt::Power2, 2 * MP_WORD_BITS * mod_words);
R2 %= modulus;
}
}
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