eprecomp.cpp

// eprecomp.cpp - written and placed in the public domain by Wei Dai

#include "pch.h"

#ifndef CRYPTOPP_IMPORTS

#include "eprecomp.h"
#include "asn.h"

NAMESPACE_BEGIN(CryptoPP)

template <class T> void DL_FixedBasePrecomputationImpl<T>::SetBase(const DL_GroupPrecomputation<Element> &group, const Element &i_base)
{
        m_base = group.NeedConversions() ? group.ConvertIn(i_base) : i_base;

        if (m_bases.empty() || !(m_base == m_bases[0]))
        {
                m_bases.resize(1);
                m_bases[0] = m_base;
        }

        if (group.NeedConversions())
                m_base = i_base;
}

template <class T> void DL_FixedBasePrecomputationImpl<T>::Precompute(const DL_GroupPrecomputation<Element> &group, unsigned int maxExpBits, unsigned int storage)
{
        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] = group.GetGroup().ScalarMultiply(m_bases[i-1], m_exponentBase);
}

template <class T> void DL_FixedBasePrecomputationImpl<T>::Load(const DL_GroupPrecomputation<Element> &group, BufferedTransformation &bt)
{
        BERSequenceDecoder seq(bt);
        word32 version;
        BERDecodeUnsigned<word32>(seq, version, INTEGER, 1, 1);
        m_exponentBase.BERDecode(seq);
        m_windowSize = m_exponentBase.BitCount() - 1;
        m_bases.clear();
        while (!seq.EndReached())
                m_bases.push_back(group.BERDecodeElement(seq));
        if (!m_bases.empty() && group.NeedConversions())
                m_base = group.ConvertOut(m_bases[0]);
        seq.MessageEnd();
}

template <class T> void DL_FixedBasePrecomputationImpl<T>::Save(const DL_GroupPrecomputation<Element> &group, BufferedTransformation &bt) const
{
        DERSequenceEncoder seq(bt);
        DEREncodeUnsigned<word32>(seq, 1);      // version
        m_exponentBase.DEREncode(seq);
        for (unsigned i=0; i<m_bases.size(); i++)
                group.DEREncodeElement(seq, m_bases[i]);
        seq.MessageEnd();
}

template <class T> void DL_FixedBasePrecomputationImpl<T>::PrepareCascade(const DL_GroupPrecomputation<Element> &i_group, std::vector<BaseAndExponent<Element> > &eb, const Integer &exponent) const
{
        const AbstractGroup<T> &group = i_group.GetGroup();

        Integer r, q, e = exponent;
        bool fastNegate = 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>(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> T DL_FixedBasePrecomputationImpl<T>::Exponentiate(const DL_GroupPrecomputation<Element> &group, const Integer &exponent) const
{
        std::vector<BaseAndExponent<Element> > eb;      // array of segments of the exponent and precalculated bases
        eb.reserve(m_bases.size());
        PrepareCascade(group, eb, exponent);
        return group.ConvertOut(GeneralCascadeMultiplication<Element>(group.GetGroup(), eb.begin(), eb.end()));
}

template <class T> T 
        DL_FixedBasePrecomputationImpl<T>::CascadeExponentiate(const DL_GroupPrecomputation<Element> &group, const Integer &exponent, 
                const DL_FixedBasePrecomputation<T> &i_pc2, const Integer &exponent2) const
{
        std::vector<BaseAndExponent<Element> > eb;      // array of segments of the exponent and precalculated bases
        const DL_FixedBasePrecomputationImpl<T> &pc2 = static_cast<const DL_FixedBasePrecomputationImpl<T> &>(i_pc2);
        eb.reserve(m_bases.size() + pc2.m_bases.size());
        PrepareCascade(group, eb, exponent);
        pc2.PrepareCascade(group, eb, exponent2);
        return group.ConvertOut(GeneralCascadeMultiplication<Element>(group.GetGroup(), eb.begin(), eb.end()));
}

NAMESPACE_END

#endif

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