gf2_32.cpp

// gf2_32.cpp - written and placed in the public domain by Wei Dai

#include "pch.h"
#include "misc.h"
#include "gf2_32.h"

NAMESPACE_BEGIN(CryptoPP)

GF2_32::Element GF2_32::Multiply(Element a, Element b) const
{
        word32 table[4];
        table[0] = 0;
        table[1] = m_modulus;
        if (a & 0x80000000)
        {
                table[2] = m_modulus ^ (a<<1);
                table[3] = a<<1;
        }
        else
        {
                table[2] = a<<1;
                table[3] = m_modulus ^ (a<<1);
        }

#ifdef FAST_ROTATE
        b = rotrFixed(b, 30U);
        word32 result = table[b&2];

        for (int i=29; i>=0; --i)
        {
                b = rotlFixed(b, 1U);
                result = (result<<1) ^ table[(b&2) + (result>>31)];
        }

        return (b&1) ? result ^ a : result;
#else
        word32 result = table[(b>>30) & 2];

        for (int i=29; i>=0; --i)
                result = (result<<1) ^ table[((b>>i)&2) + (result>>31)];

        return (b&1) ? result ^ a : result;
#endif
}

GF2_32::Element GF2_32::MultiplicativeInverse(Element a) const
{
        if (a <= 1)             // 1 is a special case
                return a;

        // warning - don't try to adapt this algorithm for another situation
        word32 g0=m_modulus, g1=a, g2=a;
        word32 v0=0, v1=1, v2=1;

        assert(g1);

        while (!(g2 & 0x80000000))
        {
                g2 <<= 1;
                v2 <<= 1;
        }

        g2 <<= 1;
        v2 <<= 1;

        g0 ^= g2;
        v0 ^= v2;

        while (g0 != 1)
        {
                if (g1 < g0 || ((g0^g1) < g0 && (g0^g1) < g1))
                {
                        assert(BitPrecision(g1) <= BitPrecision(g0));
                        g2 = g1;
                        v2 = v1;
                }
                else
                {
                        assert(BitPrecision(g1) > BitPrecision(g0));
                        g2 = g0; g0 = g1; g1 = g2;
                        v2 = v0; v0 = v1; v1 = v2;
                }

                while ((g0^g2) >= g2)
                {
                        assert(BitPrecision(g0) > BitPrecision(g2));
                        g2 <<= 1;
                        v2 <<= 1;
                }

                assert(BitPrecision(g0) == BitPrecision(g2));
                g0 ^= g2;
                v0 ^= v2;
        }

        return v0;
}

NAMESPACE_END

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