1// eprecomp.cpp - written and placed in the public domain by Wei Dai
2
3#include "pch.h"
4
5#ifndef CRYPTOPP_IMPORTS
6
7#include "eprecomp.h"
8#include "asn.h"
9
10NAMESPACE_BEGIN(CryptoPP)
11
12template <class T> void DL_FixedBasePrecomputationImpl<T>::SetBase(const DL_GroupPrecomputation<Element> &group, const Element &i_base)
13{
14	m_base = group.NeedConversions() ? group.ConvertIn(i_base) : i_base;
15
16	if (m_bases.empty() || !(m_base == m_bases[0]))
17	{
18		m_bases.resize(1);
19		m_bases[0] = m_base;
20	}
21
22	if (group.NeedConversions())
23		m_base = i_base;
24}
25
26template <class T> void DL_FixedBasePrecomputationImpl<T>::Precompute(const DL_GroupPrecomputation<Element> &group, unsigned int maxExpBits, unsigned int storage)
27{
28	assert(m_bases.size() > 0);
29	assert(storage <= maxExpBits);
30
31	if (storage > 1)
32	{
33		m_windowSize = (maxExpBits+storage-1)/storage;
34		m_exponentBase = Integer::Power2(m_windowSize);
35	}
36
37	m_bases.resize(storage);
38	for (unsigned i=1; i<storage; i++)
39		m_bases[i] = group.GetGroup().ScalarMultiply(m_bases[i-1], m_exponentBase);
40}
41
42template <class T> void DL_FixedBasePrecomputationImpl<T>::Load(const DL_GroupPrecomputation<Element> &group, BufferedTransformation &bt)
43{
44	BERSequenceDecoder seq(bt);
45	word32 version;
46	BERDecodeUnsigned<word32>(seq, version, INTEGER, 1, 1);
47	m_exponentBase.BERDecode(seq);
48	m_windowSize = m_exponentBase.BitCount() - 1;
49	m_bases.clear();
50	while (!seq.EndReached())
51		m_bases.push_back(group.BERDecodeElement(seq));
52	if (!m_bases.empty() && group.NeedConversions())
53		m_base = group.ConvertOut(m_bases[0]);
54	seq.MessageEnd();
55}
56
57template <class T> void DL_FixedBasePrecomputationImpl<T>::Save(const DL_GroupPrecomputation<Element> &group, BufferedTransformation &bt) const
58{
59	DERSequenceEncoder seq(bt);
60	DEREncodeUnsigned<word32>(seq, 1);	// version
61	m_exponentBase.DEREncode(seq);
62	for (unsigned i=0; i<m_bases.size(); i++)
63		group.DEREncodeElement(seq, m_bases[i]);
64	seq.MessageEnd();
65}
66
67template <class T> void DL_FixedBasePrecomputationImpl<T>::PrepareCascade(const DL_GroupPrecomputation<Element> &i_group, std::vector<BaseAndExponent<Element> > &eb, const Integer &exponent) const
68{
69	const AbstractGroup<T> &group = i_group.GetGroup();
70
71	Integer r, q, e = exponent;
72	bool fastNegate = group.InversionIsFast() && m_windowSize > 1;
73	unsigned int i;
74
75	for (i=0; i+1<m_bases.size(); i++)
76	{
77		Integer::DivideByPowerOf2(r, q, e, m_windowSize);
78		std::swap(q, e);
79		if (fastNegate && r.GetBit(m_windowSize-1))
80		{
81			++e;
82			eb.push_back(BaseAndExponent<Element>(group.Inverse(m_bases[i]), m_exponentBase - r));
83		}
84		else
85			eb.push_back(BaseAndExponent<Element>(m_bases[i], r));
86	}
87	eb.push_back(BaseAndExponent<Element>(m_bases[i], e));
88}
89
90template <class T> T DL_FixedBasePrecomputationImpl<T>::Exponentiate(const DL_GroupPrecomputation<Element> &group, const Integer &exponent) const
91{
92	std::vector<BaseAndExponent<Element> > eb;	// array of segments of the exponent and precalculated bases
93	eb.reserve(m_bases.size());
94	PrepareCascade(group, eb, exponent);
95	return group.ConvertOut(GeneralCascadeMultiplication<Element>(group.GetGroup(), eb.begin(), eb.end()));
96}
97
98template <class T> T
99	DL_FixedBasePrecomputationImpl<T>::CascadeExponentiate(const DL_GroupPrecomputation<Element> &group, const Integer &exponent,
100		const DL_FixedBasePrecomputation<T> &i_pc2, const Integer &exponent2) const
101{
102	std::vector<BaseAndExponent<Element> > eb;	// array of segments of the exponent and precalculated bases
103	const DL_FixedBasePrecomputationImpl<T> &pc2 = static_cast<const DL_FixedBasePrecomputationImpl<T> &>(i_pc2);
104	eb.reserve(m_bases.size() + pc2.m_bases.size());
105	PrepareCascade(group, eb, exponent);
106	pc2.PrepareCascade(group, eb, exponent2);
107	return group.ConvertOut(GeneralCascadeMultiplication<Element>(group.GetGroup(), eb.begin(), eb.end()));
108}
109
110NAMESPACE_END
111
112#endif
113