100 lines
1.6 KiB
C++
100 lines
1.6 KiB
C++
// gf2_32.cpp - written and placed in the public domain by Wei Dai
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#include "pch.h"
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#include "misc.h"
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#include "gf2_32.h"
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NAMESPACE_BEGIN(CryptoPP)
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GF2_32::Element GF2_32::Multiply(Element a, Element b) const
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{
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word32 table[4];
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table[0] = 0;
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table[1] = m_modulus;
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if (a & 0x80000000)
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{
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table[2] = m_modulus ^ (a<<1);
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table[3] = a<<1;
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}
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else
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{
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table[2] = a<<1;
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table[3] = m_modulus ^ (a<<1);
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}
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#if CRYPTOPP_FAST_ROTATE(32)
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b = rotrFixed(b, 30U);
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word32 result = table[b&2];
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for (int i=29; i>=0; --i)
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{
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b = rotlFixed(b, 1U);
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result = (result<<1) ^ table[(b&2) + (result>>31)];
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}
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return (b&1) ? result ^ a : result;
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#else
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word32 result = table[(b>>30) & 2];
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for (int i=29; i>=0; --i)
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result = (result<<1) ^ table[((b>>i)&2) + (result>>31)];
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return (b&1) ? result ^ a : result;
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#endif
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}
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GF2_32::Element GF2_32::MultiplicativeInverse(Element a) const
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{
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if (a <= 1) // 1 is a special case
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return a;
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// warning - don't try to adapt this algorithm for another situation
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word32 g0=m_modulus, g1=a, g2=a;
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word32 v0=0, v1=1, v2=1;
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assert(g1);
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while (!(g2 & 0x80000000))
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{
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g2 <<= 1;
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v2 <<= 1;
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}
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g2 <<= 1;
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v2 <<= 1;
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g0 ^= g2;
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v0 ^= v2;
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while (g0 != 1)
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{
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if (g1 < g0 || ((g0^g1) < g0 && (g0^g1) < g1))
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{
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assert(BitPrecision(g1) <= BitPrecision(g0));
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g2 = g1;
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v2 = v1;
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}
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else
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{
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assert(BitPrecision(g1) > BitPrecision(g0));
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g2 = g0; g0 = g1; g1 = g2;
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v2 = v0; v0 = v1; v1 = v2;
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}
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while ((g0^g2) >= g2)
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{
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assert(BitPrecision(g0) > BitPrecision(g2));
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g2 <<= 1;
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v2 <<= 1;
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}
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assert(BitPrecision(g0) == BitPrecision(g2));
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g0 ^= g2;
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v0 ^= v2;
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}
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return v0;
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}
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NAMESPACE_END
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