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/*
* Modular Reducer
* (C) 1999-2011 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/reducer.h>
namespace Botan {
/*
* Modular_Reducer Constructor
*/
Modular_Reducer::Modular_Reducer(const BigInt& mod)
{
if(mod <= 0)
throw Invalid_Argument("Modular_Reducer: modulus must be positive");
m_modulus = mod;
m_mod_words = m_modulus.sig_words();
m_modulus_2 = Botan::square(m_modulus);
m_mu = BigInt::power_of_2(2 * BOTAN_MP_WORD_BITS * m_mod_words) / m_modulus;
}
/*
* Barrett Reduction
*/
BigInt Modular_Reducer::reduce(const BigInt& x) const
{
if(m_mod_words == 0)
throw Invalid_State("Modular_Reducer: Never initalized");
const size_t x_sw = x.sig_words();
if(x_sw < m_mod_words || x.cmp(m_modulus, false) < 0)
{
if(x.is_negative())
return x + m_modulus; // make positive
return x;
}
else if(x.cmp(m_modulus_2, false) < 0)
{
secure_vector<word> ws;
BigInt t1(x.data() + m_mod_words - 1, x_sw - (m_mod_words - 1));
t1.mul(m_mu, ws);
t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words + 1));
t1.mul(m_modulus, ws);
t1.mask_bits(BOTAN_MP_WORD_BITS * (m_mod_words + 1));
t1.rev_sub(x.data(), std::min(x_sw, m_mod_words + 1), ws);
if(t1.is_negative())
{
t1 += BigInt::power_of_2(BOTAN_MP_WORD_BITS * (m_mod_words + 1));
}
t1.reduce_below(m_modulus, ws);
if(x.is_positive())
return t1;
else
return (m_modulus - t1);
}
else
{
// too big, fall back to normal division
return (x % m_modulus);
}
}
}
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