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external/crypto++-5.61/gf2_32.cpp

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+// 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);
+	}
+
+#if CRYPTOPP_FAST_ROTATE(32)
+	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