algebra.cpp

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/* algebra.cpp                                
 *
 * Copyright (C) 2003 Sawtooth Consulting Ltd.
 *
 * This file is part of yaSSL.
 *
 * yaSSL is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * yaSSL is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */

/* based on Wei Dai's algebra.cpp from CryptoPP */

#include "runtime.hpp"
#include "algebra.hpp"
#include "vector.hpp"   // mySTL::vector (simple)


namespace TaoCrypt {


const Integer& AbstractGroup::Double(const Element &a) const
{
    return Add(a, a);
}

const Integer& AbstractGroup::Subtract(const Element &a, const Element &b) const
{
    // make copy of a in case Inverse() overwrites it
    Element a1(a);
    return Add(a1, Inverse(b));
}

Integer& AbstractGroup::Accumulate(Element &a, const Element &b) const
{
    return a = Add(a, b);
}

Integer& AbstractGroup::Reduce(Element &a, const Element &b) const
{
    return a = Subtract(a, b);
}

const Integer& AbstractRing::Square(const Element &a) const
{
    return Multiply(a, a);
}


const Integer& AbstractRing::Divide(const Element &a, const Element &b) const
{
    // make copy of a in case MultiplicativeInverse() overwrites it
    Element a1(a);
    return Multiply(a1, MultiplicativeInverse(b));
}


const Integer& AbstractEuclideanDomain::Mod(const Element &a,
                                            const Element &b) const
{
    Element q;
    DivisionAlgorithm(result, q, a, b);
    return result;
}

const Integer& AbstractEuclideanDomain::Gcd(const Element &a,
                                            const Element &b) const
{
    mySTL::vector<Element> g(3);
    g[0]= b;
    g[1]= a;
    unsigned int i0=0, i1=1, i2=2;

    while (!Equal(g[i1], this->Identity()))
    {
        g[i2] = Mod(g[i0], g[i1]);
        unsigned int t = i0; i0 = i1; i1 = i2; i2 = t;
    }

    return result = g[i0];
}


Integer AbstractGroup::ScalarMultiply(const Element &base,
                                      const Integer &exponent) const
{
    Element result;
    SimultaneousMultiply(&result, base, &exponent, 1);
    return result;
}


Integer AbstractGroup::CascadeScalarMultiply(const Element &x,
                  const Integer &e1, const Element &y, const Integer &e2) const
{
    const unsigned expLen = max(e1.BitCount(), e2.BitCount());
    if (expLen==0)
        return Identity();

    const unsigned w = (expLen <= 46 ? 1 : (expLen <= 260 ? 2 : 3));
    const unsigned tableSize = 1<<w;
    mySTL::vector<Element> powerTable(tableSize << w);

    powerTable[1] = x;
    powerTable[tableSize] = y;
    if (w==1)
        powerTable[3] = Add(x,y);
    else
    {
        powerTable[2] = Double(x);
        powerTable[2*tableSize] = Double(y);

        unsigned i, j;

        for (i=3; i<tableSize; i+=2)
            powerTable[i] = Add(powerTable[i-2], powerTable[2]);
        for (i=1; i<tableSize; i+=2)
            for (j=i+tableSize; j<(tableSize<<w); j+=tableSize)
                powerTable[j] = Add(powerTable[j-tableSize], y);

        for (i=3*tableSize; i<(tableSize<<w); i+=2*tableSize)
            powerTable[i] = Add(powerTable[i-2*tableSize],
            powerTable[2*tableSize]);
        for (i=tableSize; i<(tableSize<<w); i+=2*tableSize)
            for (j=i+2; j<i+tableSize; j+=2)
                powerTable[j] = Add(powerTable[j-1], x);
    }

    Element result;
    unsigned power1 = 0, power2 = 0, prevPosition = expLen-1;
    bool firstTime = true;

    for (int i = expLen-1; i>=0; i--)
    {
        power1 = 2*power1 + e1.GetBit(i);
        power2 = 2*power2 + e2.GetBit(i);

        if (i==0 || 2*power1 >= tableSize || 2*power2 >= tableSize)
        {
            unsigned squaresBefore = prevPosition-i;
            unsigned squaresAfter = 0;
            prevPosition = i;
            while ((power1 || power2) && power1%2 == 0 && power2%2==0)
            {
                power1 /= 2;
                power2 /= 2;
                squaresBefore--;
                squaresAfter++;
            }
            if (firstTime)
            {
                result = powerTable[(power2<<w) + power1];
                firstTime = false;
            }
            else
            {
                while (squaresBefore--)
                result = Double(result);
                if (power1 || power2)
                    Accumulate(result, powerTable[(power2<<w) + power1]);
            }
            while (squaresAfter--)
                result = Double(result);
            power1 = power2 = 0;
        }
    }
    return result;
}


struct WindowSlider
{
    WindowSlider(const Integer &exp, bool fastNegate,
                 unsigned int windowSizeIn=0)
        : exp(exp), windowModulus(Integer::One()), windowSize(windowSizeIn),
          windowBegin(0), fastNegate(fastNegate), firstTime(true),
          finished(false)
    {
        if (windowSize == 0)
        {
            unsigned int expLen = exp.BitCount();
            windowSize = expLen <= 17 ? 1 : (expLen <= 24 ? 2 : 
                (expLen <= 70 ? 3 : (expLen <= 197 ? 4 : (expLen <= 539 ? 5 : 
                (expLen <= 1434 ? 6 : 7)))));
        }
        windowModulus <<= windowSize;
    }

    void FindNextWindow()
    {
        unsigned int expLen = exp.WordCount() * WORD_BITS;
        unsigned int skipCount = firstTime ? 0 : windowSize;
        firstTime = false;
        while (!exp.GetBit(skipCount))
        {
            if (skipCount >= expLen)
            {
                finished = true;
                return;
            }
            skipCount++;
        }

        exp >>= skipCount;
        windowBegin += skipCount;
        expWindow = exp % (1 << windowSize);

        if (fastNegate && exp.GetBit(windowSize))
        {
            negateNext = true;
            expWindow = (1 << windowSize) - expWindow;
            exp += windowModulus;
        }
        else
            negateNext = false;
    }

    Integer exp, windowModulus;
    unsigned int windowSize, windowBegin, expWindow;
    bool fastNegate, negateNext, firstTime, finished;
};


void AbstractGroup::SimultaneousMultiply(Integer *results, const Integer &base,
                          const Integer *expBegin, unsigned int expCount) const
{
    mySTL::vector<mySTL::vector<Element> > buckets(expCount);
    mySTL::vector<WindowSlider> exponents;
    exponents.reserve(expCount);
    unsigned int i;

    for (i=0; i<expCount; i++)
    {
        assert(expBegin->NotNegative());
        exponents.push_back(WindowSlider(*expBegin++, InversionIsFast(), 0));
        exponents[i].FindNextWindow();
        buckets[i].resize(1<<(exponents[i].windowSize-1), Identity());
    }

    unsigned int expBitPosition = 0;
    Element g = base;
    bool notDone = true;

    while (notDone)
    {
        notDone = false;
        for (i=0; i<expCount; i++)
        {
            if (!exponents[i].finished && expBitPosition == 
                 exponents[i].windowBegin)
            {
                Element &bucket = buckets[i][exponents[i].expWindow/2];
                if (exponents[i].negateNext)
                    Accumulate(bucket, Inverse(g));
                else
                    Accumulate(bucket, g);
                exponents[i].FindNextWindow();
            }
            notDone = notDone || !exponents[i].finished;
        }

        if (notDone)
        {
            g = Double(g);
            expBitPosition++;
        }
    }

    for (i=0; i<expCount; i++)
    {
        Element &r = *results++;
        r = buckets[i][buckets[i].size()-1];
        if (buckets[i].size() > 1)
        {
            for (int j = buckets[i].size()-2; j >= 1; j--)
            {
                Accumulate(buckets[i][j], buckets[i][j+1]);
                Accumulate(r, buckets[i][j]);
            }
            Accumulate(buckets[i][0], buckets[i][1]);
            r = Add(Double(r), buckets[i][0]);
        }
    }
}

Integer AbstractRing::Exponentiate(const Element &base,
                                   const Integer &exponent) const
{
    Element result;
    SimultaneousExponentiate(&result, base, &exponent, 1);
    return result;
}


Integer AbstractRing::CascadeExponentiate(const Element &x,
                  const Integer &e1, const Element &y, const Integer &e2) const
{
    return MultiplicativeGroup().AbstractGroup::CascadeScalarMultiply(
                x, e1, y, e2);
}


void AbstractRing::SimultaneousExponentiate(Integer *results,
                                            const Integer &base,
                         const Integer *exponents, unsigned int expCount) const
{
    MultiplicativeGroup().AbstractGroup::SimultaneousMultiply(results, base,
                                                          exponents, expCount);
}


} // namespace


#ifdef HAVE_EXPLICIT_TEMPLATE_INSTANTIATION
namespace mySTL {
template TaoCrypt::WindowSlider* uninit_copy<TaoCrypt::WindowSlider*, TaoCrypt::WindowSlider*>(TaoCrypt::WindowSlider*, TaoCrypt::WindowSlider*, TaoCrypt::WindowSlider*);
template void destroy<TaoCrypt::WindowSlider*>(TaoCrypt::WindowSlider*, TaoCrypt::WindowSlider*);
}
#endif