eprecomp.cpp

// eprecomp.cpp - originally written and placed in the public domain by Wei Dai
 
#include "pch.h"
 
#ifndef CRYPTOPP_IMPORTS
 
#include "eprecomp.h"
#include "integer.h"
#include "algebra.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)
{
    CRYPTOPP_ASSERT(m_bases.size() > 0);
    CRYPTOPP_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