eprecomp.cpp
// eprecomp.cpp - 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)
{
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
Integer::GetBit
bool GetBit(size_t i) const
Provides the i-th bit of the Integer.
Definition: integer.cpp:3043
AbstractGroup
Abstract group.
Definition: algebra.h:26
DERSequenceEncoder
DER Sequence Encoder.
Definition: asn.h:305
BufferedTransformation
Interface for buffered transformations.
Definition: cryptlib.h:1352
DL_FixedBasePrecomputation
Definition: eprecomp.h:35
Integer::DivideByPowerOf2
static void DivideByPowerOf2(Integer &r, Integer &q, const Integer &a, unsigned int n)
returns same result as Divide(r, q, a, Power2(n)), but faster
Definition: integer.cpp:4001
Integer::Power2
static Integer Power2(size_t e)
Exponentiates to a power of 2.
Definition: integer.cpp:2995
BaseAndExponent
Base and exponent.
Definition: algebra.h:249
DL_GroupPrecomputation
Definition: eprecomp.h:17
eprecomp.h
Classes for precomputation in a group.
asn.h
Classes and functions for working with ANS.1 objects.
DL_FixedBasePrecomputationImpl
Definition: eprecomp.h:55
algebra.h
Classes for performing mathematics over different fields.
CryptoPP
Crypto++ library namespace.
AbstractGroup::Inverse
virtual const Element & Inverse(const Element &a) const =0
Inverts the element in the group.
integer.h
Multiple precision integer with arithmetic operations.
AbstractGroup::InversionIsFast
virtual bool InversionIsFast() const
Determine if inversion is fast.
Definition: algebra.h:57
Integer
Multiple precision integer with arithmetic operations.
Definition: integer.h:45
BERSequenceDecoder
BER Sequence Decoder.
Definition: asn.h:295