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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>
#include <botan/internal/mp_core.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 * 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");
if(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)
{
BigInt t1 = x;
t1.set_sign(BigInt::Positive);
t1 >>= (MP_WORD_BITS * (m_mod_words - 1));
t1 *= m_mu;
t1 >>= (MP_WORD_BITS * (m_mod_words + 1));
t1 *= m_modulus;
t1.mask_bits(MP_WORD_BITS * (m_mod_words + 1));
BigInt t2 = x;
t2.set_sign(BigInt::Positive);
t2.mask_bits(MP_WORD_BITS * (m_mod_words + 1));
t2 -= t1;
if(t2.is_negative())
{
t2 += BigInt::power_of_2(MP_WORD_BITS * (m_mod_words + 1));
}
while(t2 >= m_modulus)
t2 -= m_modulus;
if(x.is_positive())
return t2;
else
return (m_modulus - t2);
}
else
{
// too big, fall back to normal division
return (x % m_modulus);
}
}
}
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