#include <reducer.h>

Public Member Functions
BigInt	cube (const BigInt &x) const

const BigInt &	get_modulus () const

bool	initialized () const

	Modular_Reducer ()

	Modular_Reducer (const BigInt &mod)

BigInt	multiply (const BigInt &x, const BigInt &y) const

BigInt	reduce (const BigInt &x) const

BigInt	square (const BigInt &x) const

Detailed Description

Modular Reducer (using Barrett's technique)

Definition at line 18 of file reducer.h.

Constructor & Destructor Documentation

Botan::Modular_Reducer::Modular_Reducer ( )

inline

Definition at line 52 of file reducer.h.

52 { m_mod_words = 0; }

Botan::Modular_Reducer::Modular_Reducer ( const BigInt & mod )

explicit

Definition at line 16 of file reducer.cpp.

References Botan::MP_WORD_BITS, Botan::BigInt::power_of_2(), Botan::BigInt::sig_words(), and Botan::square().

    {
    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;
    }

Member Function Documentation

BigInt Botan::Modular_Reducer::cube ( const BigInt & x ) const

inline

Cube mod p

Parameters

x

Returns: (x * x * x) % p

Definition at line 47 of file reducer.h.

References Botan::square().

48 { return multiply(x, this->square(x)); }

Botan::Modular_Reducer::multiply

BigInt multiply(const BigInt &x, const BigInt &y) const

Definition: reducer.h:31

Botan::Modular_Reducer::square

BigInt square(const BigInt &x) const

Definition: reducer.h:39

const BigInt& Botan::Modular_Reducer::get_modulus ( ) const

inline

Definition at line 21 of file reducer.h.

21 { return m_modulus; }

bool Botan::Modular_Reducer::initialized ( ) const

inline

Definition at line 50 of file reducer.h.

Referenced by Botan::Blinder::blind(), and Botan::Blinder::unblind().

50 { return (m_mod_words != 0); }

BigInt Botan::Modular_Reducer::multiply	(	const BigInt &	x,
		const BigInt &	y
	)		const

inline

Multiply mod p

Parameters

x
y

Returns: (x * y) % p

Definition at line 31 of file reducer.h.

Referenced by Botan::Blinder::blind(), botan_mp_mod_mul(), Botan::Fixed_Window_Exponentiator::execute(), Botan::ressol(), Botan::Fixed_Window_Exponentiator::set_base(), and Botan::Blinder::unblind().

32 { return reduce(x * y); }

Botan::Modular_Reducer::reduce

BigInt reduce(const BigInt &x) const

Definition: reducer.cpp:32

BigInt Botan::Modular_Reducer::reduce ( const BigInt & x ) const

Definition at line 32 of file reducer.cpp.

References Botan::BigInt::cmp(), Botan::BigInt::is_negative(), Botan::BigInt::is_positive(), Botan::BigInt::mask_bits(), Botan::MP_WORD_BITS, Botan::BigInt::Positive, Botan::BigInt::power_of_2(), and Botan::BigInt::set_sign().

Referenced by Botan::Montgomery_Exponentiator::Montgomery_Exponentiator(), and Botan::Montgomery_Exponentiator::set_base().

    {
    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);
       }
    }