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eprecomp.cpp

#include "pch.h"
#include "eprecomp.h"

NAMESPACE_BEGIN(CryptoPP)

template <class T> void ExponentiationPrecomputation<T>::SetGroupAndBase(const Group &group, const Element &base)
{
        m_group = &group;
        m_bases.resize(1);
        m_bases[0] = base;
}

template <class T> void ExponentiationPrecomputation<T>::Precompute(unsigned int maxExpBits, unsigned int storage)
{
        assert(m_group != NULL);
        assert(m_bases.size() > 0);
        assert(storage <= maxExpBits);

        if (storage > 1)
        {
                m_windowSize = (maxExpBits+storage-1)/storage;
                m_exponentBase = Integer::Power2(m_windowSize);
        }

        m_bases.resize(storage);
        for (unsigned i=1; i<storage; i++)
                m_bases[i] = m_group->ScalarMultiply(m_bases[i-1], m_exponentBase);
}

template <class T> void ExponentiationPrecomputation<T>::PrepareCascade(std::vector<BaseAndExponent<Element> > &eb, const Integer &exponent) const
{
        Integer r, q, e = exponent;
        bool fastNegate = m_group->InversionIsFast() && m_windowSize > 1;
        unsigned int i;

        for (i=0; i+1<m_bases.size(); i++)
        {
                Integer::DivideByPowerOf2(r, q, e, m_windowSize);
                std::swap(q, e);
                if (fastNegate && r.GetBit(m_windowSize-1))
                {
                        ++e;
                        eb.push_back(BaseAndExponent<Element>(m_group->Inverse(m_bases[i]), m_exponentBase - r));
                }
                else
                        eb.push_back(BaseAndExponent<Element>(m_bases[i], r));
        }
        eb.push_back(BaseAndExponent<Element>(m_bases[i], e));
}

template <class T> ExponentiationPrecomputation<T>::Element ExponentiationPrecomputation<T>::Exponentiate(const Integer &exponent) const
{
        std::vector<BaseAndExponent<Element> > eb;      // array of segments of the exponent and precalculated bases
        eb.reserve(m_bases.size());
        PrepareCascade(eb, exponent);
        return GeneralCascadeMultiplication<Element>(*m_group, eb.begin(), eb.end());
}

template <class T> T 
        ExponentiationPrecomputation<T>::CascadeExponentiate(const Integer &exponent, 
                const ExponentiationPrecomputation<T> &pc2, const Integer &exponent2) const
{
        std::vector<BaseAndExponent<Element> > eb;      // array of segments of the exponent and precalculated bases
        eb.reserve(m_bases.size() + pc2.m_bases.size());
        PrepareCascade(eb, exponent);
        pc2.PrepareCascade(eb, exponent2);
        return GeneralCascadeMultiplication<Element>(*m_group, eb.begin(), eb.end());
}

NAMESPACE_END

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