gf2_32.cpp

// gf2_32.cpp - originally 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);
    }
 
#if CRYPTOPP_FAST_ROTATE(32)
    b = rotrConstant<30>(b);
    word32 result = table[b&2];
 
    for (int i=29; i>=0; --i)
    {
        b = rotlConstant<1>(b);
        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;
 
    CRYPTOPP_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))
        {
            CRYPTOPP_ASSERT(BitPrecision(g1) <= BitPrecision(g0));
            g2 = g1;
            v2 = v1;
        }
        else
        {
            CRYPTOPP_ASSERT(BitPrecision(g1) > BitPrecision(g0));
            g2 = g0; g0 = g1; g1 = g2;
            v2 = v0; v0 = v1; v1 = v2;
        }
 
        while ((g0^g2) >= g2)
        {
            CRYPTOPP_ASSERT(BitPrecision(g0) > BitPrecision(g2));
            g2 <<= 1;
            v2 <<= 1;
        }
 
        CRYPTOPP_ASSERT(BitPrecision(g0) == BitPrecision(g2));
        g0 ^= g2;
        v0 ^= v2;
    }
 
    return v0;
}
 
NAMESPACE_END