From b21162cf8e06f40baa1f58be6a8c17435cebc34d Mon Sep 17 00:00:00 2001
From: weidai <weidai@57ff6487-cd31-0410-9ec3-f628ee90f5f0>
Date: Fri, 4 Oct 2002 17:31:41 +0000
Subject: Initial revision

git-svn-id: svn://svn.code.sf.net/p/cryptopp/code/trunk/c5@2 57ff6487-cd31-0410-9ec3-f628ee90f5f0
---
 eprecomp.cpp | 107 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100644 eprecomp.cpp

(limited to 'eprecomp.cpp')

diff --git a/eprecomp.cpp b/eprecomp.cpp
new file mode 100644
index 0000000..f9878b9
--- /dev/null
+++ b/eprecomp.cpp
@@ -0,0 +1,107 @@
+// eprecomp.cpp - written and placed in the public domain by Wei Dai
+
+#include "pch.h"
+#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
-- 
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