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# eprecomp.cpp

    00001 // eprecomp.cpp - written and placed in the public domain by Wei Dai
    00002
    00003 #include "pch.h"
    00004
    00005 #ifndef CRYPTOPP_IMPORTS
    00006
    00007 #include "eprecomp.h"
    00008 #include "asn.h"
    00009
    00010 NAMESPACE_BEGIN(CryptoPP)
    00011
    00012 template <class T> void DL_FixedBasePrecomputationImpl<T>::SetBase(const DL_GroupPrecomputation<Element> &group, const Element &i_base)
    00013 {
    00014         m_base = group.NeedConversions() ? group.ConvertIn(i_base) : i_base;
    00015
    00016         if (m_bases.empty() || !(m_base == m_bases[0]))
    00017         {
    00018                 m_bases.resize(1);
    00019                 m_bases[0] = m_base;
    00020         }
    00021
    00022         if (group.NeedConversions())
    00023                 m_base = i_base;
    00024 }
    00025
    00026 template <class T> void DL_FixedBasePrecomputationImpl<T>::Precompute(const DL_GroupPrecomputation<Element> &group, unsigned int maxExpBits, unsigned int storage)
    00027 {
    00028         assert(m_bases.size() > 0);
    00029         assert(storage <= maxExpBits);
    00030
    00031         if (storage > 1)
    00032         {
    00033                 m_windowSize = (maxExpBits+storage-1)/storage;
    00034                 m_exponentBase = Integer::Power2(m_windowSize);
    00035         }
    00036
    00037         m_bases.resize(storage);
    00038         for (unsigned i=1; i<storage; i++)
    00039                 m_bases[i] = group.GetGroup().ScalarMultiply(m_bases[i-1], m_exponentBase);
    00040 }
    00041
    00042 template <class T> void DL_FixedBasePrecomputationImpl<T>::Load(const DL_GroupPrecomputation<Element> &group, BufferedTransformation &bt)
    00043 {
    00044         BERSequenceDecoder seq(bt);
    00045         word32 version;
    00046         BERDecodeUnsigned<word32>(seq, version, INTEGER, 1, 1);
    00047         m_exponentBase.BERDecode(seq);
    00048         m_windowSize = m_exponentBase.BitCount() - 1;
    00049         m_bases.clear();
    00050         while (!seq.EndReached())
    00051                 m_bases.push_back(group.BERDecodeElement(seq));
    00052         if (!m_bases.empty() && group.NeedConversions())
    00053                 m_base = group.ConvertOut(m_bases[0]);
    00054         seq.MessageEnd();
    00055 }
    00056
    00057 template <class T> void DL_FixedBasePrecomputationImpl<T>::Save(const DL_GroupPrecomputation<Element> &group, BufferedTransformation &bt) const
    00058 {
    00059         DERSequenceEncoder seq(bt);
    00060         DEREncodeUnsigned<word32>(seq, 1);      // version
    00061         m_exponentBase.DEREncode(seq);
    00062         for (unsigned i=0; i<m_bases.size(); i++)
    00063                 group.DEREncodeElement(seq, m_bases[i]);
    00064         seq.MessageEnd();
    00065 }
    00066
    00067 template <class T> void DL_FixedBasePrecomputationImpl<T>::PrepareCascade(const DL_GroupPrecomputation<Element> &i_group, std::vector<BaseAndExponent<Element> > &eb, const Integer &exponent) const
    00068 {
    00069         const AbstractGroup<T> &group = i_group.GetGroup();
    00070
    00071         Integer r, q, e = exponent;
    00072         bool fastNegate = group.InversionIsFast() && m_windowSize > 1;
    00073         unsigned int i;
    00074
    00075         for (i=0; i+1<m_bases.size(); i++)
    00076         {
    00077                 Integer::DivideByPowerOf2(r, q, e, m_windowSize);
    00078                 std::swap(q, e);
    00079                 if (fastNegate && r.GetBit(m_windowSize-1))
    00080                 {
    00081                         ++e;
    00082                         eb.push_back(BaseAndExponent<Element>(group.Inverse(m_bases[i]), m_exponentBase - r));
    00083                 }
    00084                 else
    00085                         eb.push_back(BaseAndExponent<Element>(m_bases[i], r));
    00086         }
    00087         eb.push_back(BaseAndExponent<Element>(m_bases[i], e));
    00088 }
    00089
    00090 template <class T> T DL_FixedBasePrecomputationImpl<T>::Exponentiate(const DL_GroupPrecomputation<Element> &group, const Integer &exponent) const
    00091 {
    00092         std::vector<BaseAndExponent<Element> > eb;      // array of segments of the exponent and precalculated bases
    00093         eb.reserve(m_bases.size());
    00094         PrepareCascade(group, eb, exponent);
    00095         return group.ConvertOut(GeneralCascadeMultiplication<Element>(group.GetGroup(), eb.begin(), eb.end()));
    00096 }
    00097
    00098 template <class T> T
    00099         DL_FixedBasePrecomputationImpl<T>::CascadeExponentiate(const DL_GroupPrecomputation<Element> &group, const Integer &exponent,
    00100                 const DL_FixedBasePrecomputation<T> &i_pc2, const Integer &exponent2) const
    00101 {
    00102         std::vector<BaseAndExponent<Element> > eb;      // array of segments of the exponent and precalculated bases
    00103         const DL_FixedBasePrecomputationImpl<T> &pc2 = static_cast<const DL_FixedBasePrecomputationImpl<T> &>(i_pc2);
    00104         eb.reserve(m_bases.size() + pc2.m_bases.size());
    00105         PrepareCascade(group, eb, exponent);
    00106         pc2.PrepareCascade(group, eb, exponent2);
    00107         return group.ConvertOut(GeneralCascadeMultiplication<Element>(group.GetGroup(), eb.begin(), eb.end()));
    00108 }
    00109
    00110 NAMESPACE_END
    00111
    00112 #endif